Move the std::vector<std::byte> tests to unit-custom-binary-type-cpp17.cpp

The develop merge kept the test case that #5809 moved from
unit-regression3.cpp in unit-custom-binary-type.cpp, but byte_binary_json
is only defined in the C++17 file now, so the C++17 build of the former
failed.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/CONTRIBUTING.md

@@ -108,9 +108,16 @@ The tests are located in [`tests/src/unit-*.cpp`](https://github.com/nlohmann/js
 are structured along the features of the library or the nature of the tests. Usually, it should be clear from the
 context which existing file needs to be extended, and only very few cases require creating new test files.
 
-When fixing a bug, edit `unit-regression3.cpp` and add a section referencing the fixed issue.
-`unit-regression2.cpp` holds the older tests; the two files exist because a single one grew large enough for the
-MinGW linker to fail relocating it, so please keep adding to the smaller file rather than growing the larger one.
+When fixing a bug, edit `unit-regression3.cpp` and add a section referencing the fixed issue. Tests that need C++17 or
+C++20 go into `unit-regression3-cpp17.cpp` or `unit-regression3-cpp20.cpp` instead, so the large file is not rebuilt
+for every C++ standard. `unit-regression2.cpp` holds the older tests; the two files exist because a single one grew
+large enough for the MinGW linker to fail relocating it, so please keep adding to the smaller file rather than growing
+the larger one.
+
+To keep compile times down, several test files are compiled together as one translation unit. This sets a few rules
+for test files, such as giving file-scope helpers file-specific names. See the README in the
+[`tests`](https://github.com/nlohmann/json/tree/develop/tests) folder for how the tests are built and what to keep in
+mind when adding them.
 
 #### Exceptions
 

.github/labeler.yml

@@ -63,7 +63,7 @@ labels:
     - "single_include/nlohmann/json_view\\.hpp"
     - "tests/src/unit-json_view.*"
     - "tests/src/json_view_test_helpers\\.hpp"
-    - "tests/src/fuzzer-parse_json_view\\.cpp"
+    - "tests/src/fuzzer-(parse_json_view|json_view_image)\\.cpp"
     - "tools/amalgamate/config_json_view\\.json"
     - "docs/mkdocs/docs/features/json_view\\.md"
     - "docs/mkdocs/docs/api/basic_json_(document|view)/.*"

.github/workflows/check_amalgamation.yml

@@ -77,13 +77,12 @@ jobs:
           cd $MAIN_DIR
 
           # Built from this PR's own tools/macro_builder/main.cpp, not a
-          # develop checkout: unlike amalgamate.py and generate_natvis.py,
-          # this tool has no other source of truth to check against (its
-          # own README documents that it must reproduce macro_scope.hpp
-          # byte for byte), so there is nothing to gain from checking out a
-          # separate copy, and doing so would make this step fail on a PR
-          # that adds support for a new dispatch table until that PR itself
-          # merges to develop.
+          # develop checkout: unlike amalgamate.py, this tool has no other
+          # source of truth to check against (its own README documents that
+          # it must reproduce macro_scope.hpp byte for byte), so there is
+          # nothing to gain from checking out a separate copy, and doing so
+          # would make this step fail on a PR that adds support for a new
+          # dispatch table until that PR itself merges to develop.
           TMPDIR=$(mktemp -d ./macro_builder_check.XXXXXX)
           c++ -std=c++11 tools/macro_builder/main.cpp -o "$TMPDIR/macro_builder"
           "$TMPDIR/macro_builder" > "$TMPDIR/paste.hpp"
@@ -146,14 +145,14 @@ jobs:
       - name: Regenerate nlohmann_json.natvis
         run: |
           cd $MAIN_DIR
-          # The template is what this check verifies the committed natvis
-          # file against, so take it from this PR rather than from the
-          # develop checkout: generate_natvis.py loads the template from its
-          # own directory, and with develop's copy a PR that changes the
-          # template could never pass. Like the macro_builder step above,
-          # this only renders PR content in a job without credentials.
-          cp tools/generate_natvis/nlohmann_json.natvis.j2 $NATVIS_TOOL_DIR/
-          python3 $NATVIS_TOOL_DIR/generate_natvis.py $MAIN_DIR
+          # The generator and its template are what this check verifies the
+          # committed natvis file against (which namespaces it lists, and
+          # what each entry looks like), so run this PR's copy rather than
+          # the develop checkout's: with develop's, a PR that changes either
+          # could never pass. generate_natvis.py loads the template from its
+          # own directory. Like the macro_builder step above, this only runs
+          # PR content in a job without credentials.
+          python3 tools/generate_natvis/generate_natvis.py $MAIN_DIR
 
       - name: Build patch and check for differences
         id: diff

.github/workflows/scorecards.yml

@@ -50,7 +50,7 @@ jobs:
           persist-credentials: false
 
       - name: "Run analysis"
-        uses: ossf/scorecard-action@2d1146689b8cda280b9bc96326124645441f03bc # v2.4.4
+        uses: ossf/scorecard-action@fa7117db83c52d787ae5a89f350999701505d78e # v2.5.0
         with:
           results_file: results.sarif
           results_format: sarif

BUILD.bazel

@@ -73,6 +73,7 @@ cc_library(
         "include/nlohmann/detail/view/edit.hpp",
         "include/nlohmann/detail/view/edit_storage.hpp",
         "include/nlohmann/detail/view/errors.hpp",
+        "include/nlohmann/detail/view/image.hpp",
         "include/nlohmann/detail/view/input.hpp",
         "include/nlohmann/detail/view/iterator.hpp",
         "include/nlohmann/detail/view/lookup.hpp",

cmake/ci.cmake

@@ -890,11 +890,11 @@ add_custom_target(ci_icpx
 # The following tests are excluded as they trigger known nvc++ 25.5 defects (not
 # library bugs); see https://github.com/nlohmann/json for tracking. Only the
 # affected language-standard variants are excluded so coverage is otherwise kept:
-#   - test-comparison_cpp20, test-comparison_legacy_cpp20
+#   - test-comparison-cpp20_cpp20, test-comparison_legacy-cpp20_cpp20
 #         miscompiles cross-type/<=> comparison (e.g. `-17 <= null`)
 #   - test-constructor1_cpp11
 #         std::initializer_list lifetime bug -> SIGSEGV
-#   - test-deserialization_cpp20
+#   - test-deserialization-cpp20_cpp20
 #         mangles the UTF-8 u8"" string literal in the char8_t (C++20) section
 add_custom_target(ci_nvhpc
     COMMAND ${CMAKE_COMMAND}
@@ -906,7 +906,7 @@ add_custom_target(ci_nvhpc
     COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_nvhpc
     # the pipes are escaped so the surrounding shell passes them to ctest verbatim
     # instead of treating them as shell pipe operators
-    COMMAND cd ${PROJECT_BINARY_DIR}/build_nvhpc && ${CMAKE_CTEST_COMMAND} --parallel ${N} --exclude-regex "test-comparison_cpp20\\|test-comparison_legacy_cpp20\\|test-constructor1_cpp11\\|test-deserialization_cpp20" --output-on-failure
+    COMMAND cd ${PROJECT_BINARY_DIR}/build_nvhpc && ${CMAKE_CTEST_COMMAND} --parallel ${N} --exclude-regex "test-comparison-cpp20_cpp20\\|test-comparison_legacy-cpp20_cpp20\\|test-constructor1_cpp11\\|test-deserialization-cpp20_cpp20" --output-on-failure
     COMMENT "Compile and test with NVIDIA HPC SDK (nvc++)"
 )
 

cmake/requirements/requirements-cppcheck.txt

@@ -1 +1 @@
-cppcheck==1.5.2
+cppcheck==1.5.3

cmake/test.cmake

@@ -109,6 +109,55 @@ function(_json_test_apply_test_properties test_target properties_target)
     endif()
 endfunction()
 
+# for internal use by _json_test_add_test() and _json_test_add_unity_batch():
+# registers the CTest test <test_name>_cpp<cxx_standard> (plus its Valgrind
+# variant), which runs the executable target <test_target> with the arguments
+# in ARGN, and applies the test properties of the test- and standard-specific
+# interface targets
+function(_json_test_register_test test_name test_target cxx_standard)
+    set(ctest_name ${test_name}_cpp${cxx_standard})
+
+    if (JSON_FastTests)
+        add_test(NAME ${ctest_name}
+            COMMAND ${test_target} ${DOCTEST_TEST_FILTER} ${ARGN}
+            WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+        )
+    else()
+        add_test(NAME ${ctest_name}
+            COMMAND ${test_target} ${DOCTEST_TEST_FILTER} ${ARGN} --no-skip
+            WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+        )
+    endif()
+    set_tests_properties(${ctest_name} PROPERTIES LABELS "all" FIXTURES_REQUIRED TEST_DATA)
+
+    # apply standard-specific test properties
+    if(TARGET _json_test_interface__cpp_${cxx_standard})
+        _json_test_apply_test_properties(${ctest_name} _json_test_interface__cpp_${cxx_standard})
+    endif()
+
+    # apply test-specific test properties
+    if(TARGET _json_test_interface_${test_name})
+        _json_test_apply_test_properties(${ctest_name} _json_test_interface_${test_name})
+    endif()
+
+    # apply test- and standard-specific test properties
+    if(TARGET _json_test_interface_${test_name}_cpp_${cxx_standard})
+        _json_test_apply_test_properties(${ctest_name}
+            _json_test_interface_${test_name}_cpp_${cxx_standard}
+        )
+    endif()
+
+    if(JSON_Valgrind)
+        add_test(NAME ${ctest_name}_valgrind
+            COMMAND ${memcheck_command} $<TARGET_FILE:${test_target}> ${DOCTEST_TEST_FILTER} ${ARGN}
+            WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+        )
+        set_tests_properties(${ctest_name}_valgrind PROPERTIES
+            LABELS "valgrind" FIXTURES_REQUIRED TEST_DATA
+        )
+    endif()
+endfunction()
+
 # for internal use by json_test_add_test_for()
 function(_json_test_add_test test_name file main cxx_standard)
     set(test_target ${test_name}_cpp${cxx_standard})
@@ -143,45 +192,7 @@ function(_json_test_add_test test_name file main cxx_standard)
         )
     endif()
 
-    if (JSON_FastTests)
-        add_test(NAME ${test_target}
-            COMMAND ${test_target} ${DOCTEST_TEST_FILTER}
-            WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
-        )
-    else()
-        add_test(NAME ${test_target}
-            COMMAND ${test_target} ${DOCTEST_TEST_FILTER} --no-skip
-            WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
-        )
-    endif()
-    set_tests_properties(${test_target} PROPERTIES LABELS "all" FIXTURES_REQUIRED TEST_DATA)
-
-    # apply standard-specific test properties
-    if(TARGET _json_test_interface__cpp_${cxx_standard})
-        _json_test_apply_test_properties(${test_target} _json_test_interface__cpp_${cxx_standard})
-    endif()
-
-    # apply test-specific test properties
-    if(TARGET _json_test_interface_${test_name})
-        _json_test_apply_test_properties(${test_target} _json_test_interface_${test_name})
-    endif()
-
-    # apply test- and standard-specific test properties
-    if(TARGET _json_test_interface_${test_name}_cpp_${cxx_standard})
-        _json_test_apply_test_properties(${test_target}
-            _json_test_interface_${test_name}_cpp_${cxx_standard}
-        )
-    endif()
-
-    if(JSON_Valgrind)
-        add_test(NAME ${test_target}_valgrind
-            COMMAND ${memcheck_command} $<TARGET_FILE:${test_target}> ${DOCTEST_TEST_FILTER}
-            WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
-        )
-        set_tests_properties(${test_target}_valgrind PROPERTIES
-            LABELS "valgrind" FIXTURES_REQUIRED TEST_DATA
-        )
-    endif()
+    _json_test_register_test(${test_name} ${test_target} ${cxx_standard})
 endfunction()
 
 #############################################################################
@@ -200,6 +211,14 @@ endfunction()
 # Use NAME <name> to override the filename-derived test name.
 # Use FORCE to create the test regardless of the file containing
 # JSON_HAS_CPP_<version_number>.
+#
+# Tests that depend on the C++ standard (e.g., because they use the macros
+# JSON_HAS_FILESYSTEM, JSON_HAS_RANGES, or JSON_HAS_THREE_WAY_COMPARISON)
+# should not make the whole of a large unit-foo.cpp be rebuilt for every
+# standard. Put them into a separate file unit-foo-cpp<NN>.cpp (see, e.g.,
+# unit-items-cpp17.cpp) which wraps its content in #ifdef JSON_HAS_CPP_<NN>.
+# Then, unit-foo.cpp itself contains no JSON_HAS_CPP_<NN> and is only built for
+# C++11.
 # Test targets are linked against <main>.
 # CXX_STANDARDS defaults to "11".
 #############################################################################
@@ -243,6 +262,314 @@ function(json_test_add_test_for file)
     endforeach()
 endfunction()
 
+# for internal use by json_test_add_unity_tests(): sets <result> to whether the
+# (absolute) <file> is built for <cxx_standard>; same rule as in
+# json_test_add_test_for(): C++11 always, others only if the file contains
+# JSON_HAS_CPP_<cxx_standard> or <force> is set
+function(_json_test_unity_applies file cxx_standard force result)
+    set(${result} TRUE PARENT_SCOPE)
+    if(NOT ("${cxx_standard}" STREQUAL 11 OR force))
+        file(READ ${file} file_content)
+        string(FIND "${file_content}" JSON_HAS_CPP_${cxx_standard} has_cpp_found)
+        if(${has_cpp_found} EQUAL -1)
+            set(${result} FALSE PARENT_SCOPE)
+        endif()
+    endif()
+endfunction()
+
+# for internal use by json_test_add_unity_tests(): creates the executable
+# test-unity-<batch_name>_cpp<cxx_standard> from the (absolute) source files
+# in ARGN, which are #include-d by a generated source file, and registers one
+# CTest test per source file that runs only the test cases of that file; if
+# <private> is true, the generated file defines JSON_TESTS_PRIVATE first
+function(_json_test_add_unity_batch batch_name cxx_standard main private)
+    set(batch_target test-unity-${batch_name}_cpp${cxx_standard})
+    set(batch_source ${PROJECT_BINARY_DIR}/tests/unity/${batch_target}.cpp)
+
+    set(batch_content "// generated by cmake/test.cmake; do not edit\n")
+    if(private)
+        string(APPEND batch_content "// at least one file of this batch needs access to private members of the library\n")
+        string(APPEND batch_content "#define JSON_TESTS_PRIVATE\n")
+        # the files define the macro again; mark this definition as used, as
+        # -Wunused-macros reports an unused definition when it is redefined
+        string(APPEND batch_content "#ifdef JSON_TESTS_PRIVATE\n#endif\n")
+    endif()
+    foreach(file ${ARGN})
+        string(APPEND batch_content "#include \"${file}\" // NOLINT(bugprone-suspicious-include)\n")
+    endforeach()
+
+    # only touch the generated file if it changed to keep incremental builds incremental
+    set(old_content "")
+    if(EXISTS ${batch_source})
+        file(READ ${batch_source} old_content)
+    endif()
+    if(NOT "${old_content}" STREQUAL "${batch_content}")
+        file(WRITE ${batch_source} "${batch_content}")
+    endif()
+
+    add_executable(${batch_target} ${batch_source})
+    target_link_libraries(${batch_target} PRIVATE ${main})
+    set_target_properties(${batch_target} PROPERTIES
+        CXX_STANDARD ${cxx_standard}
+        CXX_STANDARD_REQUIRED ON
+    )
+    if(TARGET _json_test_interface__cpp_${cxx_standard})
+        target_link_libraries(${batch_target} PRIVATE _json_test_interface__cpp_${cxx_standard})
+    endif()
+
+    # rebuild the batch when one of its files changes, and show the files in IDEs;
+    # files that are also built standalone (VARIANT_FILES) are left out, as
+    # HEADER_FILE_ONLY is a per-file property and would also affect those targets
+    set_source_files_properties(${batch_source} PROPERTIES OBJECT_DEPENDS "${ARGN}")
+    foreach(file ${ARGN})
+        if(NOT file IN_LIST _json_test_unity_variant_files)
+            set_source_files_properties(${file} PROPERTIES HEADER_FILE_ONLY ON)
+            target_sources(${batch_target} PRIVATE ${file})
+        endif()
+    endforeach()
+
+    foreach(file ${ARGN})
+        get_filename_component(file_basename ${file} NAME_WE)
+        string(REGEX REPLACE "unit-(.+)" "test-\\1" test_name ${file_basename})
+
+        # run only the test cases defined in this file (and in the shared
+        # make_test_data_available.hpp if the file uses it)
+        file(READ ${file} file_content)
+        set(source_filter "--source-file=*${file_basename}.cpp")
+        string(FIND "${file_content}" make_test_data_available.hpp uses_test_data)
+        if(NOT ${uses_test_data} EQUAL -1)
+            string(APPEND source_filter ",*make_test_data_available.hpp")
+        endif()
+
+        _json_test_register_test(${test_name} ${batch_target} ${cxx_standard} "${source_filter}")
+    endforeach()
+endfunction()
+
+#############################################################################
+# json_test_add_unity_tests(
+#     FILES <files>...
+#     MAIN <main>
+#     [CXX_STANDARDS <version_number>...] [FORCE]
+#     [BATCH_SIZE <size>]
+#     [GROUPS <group>...]
+#     [VARIANT_FILES <files>...])
+#
+# Like calling json_test_add_test_for(<file> MAIN <main> ...) for each of the
+# <files>, but compiles several files together to speed up the build: for each
+# C++ standard, the files are split into batches of <size> files (default: 8)
+# and each batch is built as a single executable
+#
+#     test-unity-<pool><index>_cpp<version_number>
+#
+# whose generated source file #include-s the files of the batch (so they share
+# the template instantiations of the library). The tests are still named
+# test-foo_cpp<version_number>, one per file, but run the batch executable with
+# a doctest filter that selects the test cases of that file only.
+#
+# Files are only batched with files that agree on the macros defined before
+# the library is included: files that define at most JSON_TESTS_PRIVATE (or
+# macros derived from global compile definitions) form the pools "plain" and
+# "private". All other files are added with json_test_add_test_for() as usual,
+# as are files with test-specific build settings (see
+# json_test_set_test_options()) and the files listed in the explicit
+# exclusion list below.
+# Each <group> names a list variable json_test_unity_group_<group> of test file
+# stems (file names without "unit-" and ".cpp"). The batchable files of a group
+# are compiled together (regardless of BATCH_SIZE) as one executable
+#
+#     test-unity-<group>_cpp<version_number>
+#
+# so that related tests, which instantiate the same templates, share one
+# translation unit. A group may mix the pools "plain" and "private"; if any of
+# its files needs JSON_TESTS_PRIVATE, the whole group is built with it. Files
+# that cannot be batched stay standalone even if they are listed in a group.
+# Files in no group are batched by BATCH_SIZE as described above.
+# <files> in VARIANT_FILES are also built standalone with other settings, so
+# they are not marked as header-only sources of the batch.
+#############################################################################
+
+function(json_test_add_unity_tests)
+    cmake_parse_arguments(args "FORCE" "MAIN;BATCH_SIZE" "FILES;CXX_STANDARDS;VARIANT_FILES;GROUPS" ${ARGN})
+
+    if("${args_MAIN}" STREQUAL "")
+        message(FATAL_ERROR "Required argument MAIN <main> missing.")
+    endif()
+
+    if("${args_BATCH_SIZE}" STREQUAL "")
+        set(args_BATCH_SIZE 8)
+    endif()
+
+    if("${args_CXX_STANDARDS}" STREQUAL "")
+        set(args_CXX_STANDARDS 11)
+    endif()
+
+    if(args_FORCE)
+        set(force FORCE)
+    else()
+        set(force "")
+    endif()
+
+    set(_json_test_unity_variant_files "")
+    foreach(file ${args_VARIANT_FILES})
+        get_filename_component(file ${file} ABSOLUTE)
+        list(APPEND _json_test_unity_variant_files ${file})
+    endforeach()
+
+    # files that must not be merged into a batch: unit-32bit.cpp is only built
+    # for 32bit targets, and unit-no-macro-leak.cpp checks that including the
+    # library defines no unprefixed macro, which any other file would disturb
+    set(standalone_files unit-32bit.cpp unit-no-macro-leak.cpp)
+
+    set(harmless_macros "^(DOCTEST_.*|SKIP_TESTS_FOR_.*|JSON_TEST_DEPRECATED_FUNCTIONS_DELETED|JSON_TEST_STRICT_NUL_HANDLING_ENABLED|JSON_TEST_STRINGIZE)$")
+
+    # classify the files
+    set(plain_files "")
+    set(private_files "")
+    set(standalone_abs_files "")
+    foreach(file ${args_FILES})
+        get_filename_component(file_name ${file} NAME)
+        get_filename_component(file_basename ${file} NAME_WE)
+        string(REGEX REPLACE "unit-(.+)" "test-\\1" test_name ${file_basename})
+
+        set(batchable TRUE)
+        if(file_name IN_LIST standalone_files)
+            set(batchable FALSE)
+        endif()
+        if(TARGET _json_test_interface_${test_name})
+            set(batchable FALSE)
+        endif()
+        foreach(cxx_standard ${args_CXX_STANDARDS})
+            if(TARGET _json_test_interface_${test_name}_cpp_${cxx_standard})
+                set(batchable FALSE)
+            endif()
+        endforeach()
+
+        # files that suppress GCC's -Wnoexcept around their include of the
+        # library: the suppression has no effect once another file of the
+        # batch included the library first
+        if(batchable)
+            file(READ ${file} file_content)
+            string(FIND "${file_content}" "SUPPRESS_WARNING(\"-Wnoexcept\")" suppresses_noexcept)
+            if(NOT ${suppresses_noexcept} EQUAL -1)
+                set(batchable FALSE)
+            endif()
+        endif()
+
+        set(pool plain)
+        if(batchable)
+            # collect the macros (un)defined before the library is included
+            file(READ ${file} file_content)
+            string(FIND "${file_content}" "#include <nlohmann/" include_position)
+            if(NOT ${include_position} EQUAL -1)
+                string(SUBSTRING "${file_content}" 0 ${include_position} file_content)
+            endif()
+            string(REGEX MATCHALL "(^|\n)[ \t]*#[ \t]*(define|undef)[ \t]+[A-Za-z_0-9]+" directives "${file_content}")
+            foreach(directive ${directives})
+                string(REGEX REPLACE "^.*[ \t]([A-Za-z_0-9]+)$" "\\1" macro "${directive}")
+                if(macro MATCHES "${harmless_macros}")
+                    continue()
+                elseif("${macro}" STREQUAL JSON_TESTS_PRIVATE)
+                    set(pool private)
+                else()
+                    set(batchable FALSE)
+                endif()
+            endforeach()
+        endif()
+
+        get_filename_component(file_abs ${file} ABSOLUTE)
+        if(NOT batchable)
+            list(APPEND standalone_abs_files ${file_abs})
+            json_test_add_test_for(${file} MAIN ${args_MAIN} CXX_STANDARDS ${args_CXX_STANDARDS} ${force})
+            continue()
+        endif()
+        get_filename_component(file ${file} ABSOLUTE)
+        list(APPEND ${pool}_files ${file})
+    endforeach()
+
+    # resolve the explicit groups: group_<name>_files are the (absolute) files
+    # of the group, group_<name>_private tells whether one of them is private
+    set(grouped_files "")
+    foreach(group ${args_GROUPS})
+        if(NOT DEFINED json_test_unity_group_${group})
+            message(FATAL_ERROR "Unity test group '${group}' is not defined (json_test_unity_group_${group}).")
+        endif()
+        set(group_${group}_files "")
+        set(group_${group}_private FALSE)
+        foreach(stem ${json_test_unity_group_${group}})
+            # check against the source directory, because FILES may be filtered (JSON_TestShard)
+            get_filename_component(file ${CMAKE_CURRENT_SOURCE_DIR}/src/unit-${stem}.cpp ABSOLUTE)
+            if(NOT EXISTS ${file})
+                message(FATAL_ERROR "Unity test group '${group}' lists '${stem}', but ${file} does not exist.")
+            endif()
+            if(file IN_LIST grouped_files)
+                message(FATAL_ERROR "Unity test file unit-${stem}.cpp is listed in more than one group (second: '${group}').")
+            endif()
+            list(APPEND grouped_files ${file})
+
+            if(file IN_LIST standalone_abs_files)
+                message(STATUS "Unity test group '${group}': unit-${stem}.cpp cannot be batched and stays standalone")
+            elseif(file IN_LIST plain_files)
+                list(APPEND group_${group}_files ${file})
+                list(REMOVE_ITEM plain_files ${file})
+            elseif(file IN_LIST private_files)
+                list(APPEND group_${group}_files ${file})
+                list(REMOVE_ITEM private_files ${file})
+                set(group_${group}_private TRUE)
+            endif()
+        endforeach()
+    endforeach()
+
+    foreach(cxx_standard ${args_CXX_STANDARDS})
+        if(NOT compiler_supports_cpp_${cxx_standard})
+            continue()
+        endif()
+
+        # explicit groups: one batch per group
+        foreach(group ${args_GROUPS})
+            set(batch_files "")
+            foreach(file ${group_${group}_files})
+                _json_test_unity_applies(${file} ${cxx_standard} "${force}" applies)
+                if(applies)
+                    list(APPEND batch_files ${file})
+                endif()
+            endforeach()
+            if(batch_files)
+                _json_test_add_unity_batch(${group} ${cxx_standard} ${args_MAIN} ${group_${group}_private} ${batch_files})
+            endif()
+        endforeach()
+
+        # remaining files: batches of BATCH_SIZE files per pool
+        foreach(pool plain private)
+            set(is_private FALSE)
+            if(pool STREQUAL private)
+                set(is_private TRUE)
+            endif()
+
+            set(batch_files "")
+            set(batch_count 0)
+            set(batch_index 0)
+            foreach(file ${${pool}_files})
+                _json_test_unity_applies(${file} ${cxx_standard} "${force}" applies)
+                if(NOT applies)
+                    continue()
+                endif()
+
+                list(APPEND batch_files ${file})
+                math(EXPR batch_count "${batch_count} + 1")
+                if(batch_count EQUAL args_BATCH_SIZE)
+                    _json_test_add_unity_batch(${pool}${batch_index} ${cxx_standard} ${args_MAIN} ${is_private} ${batch_files})
+                    set(batch_files "")
+                    set(batch_count 0)
+                    math(EXPR batch_index "${batch_index} + 1")
+                endif()
+            endforeach()
+            if(batch_files)
+                _json_test_add_unity_batch(${pool}${batch_index} ${cxx_standard} ${args_MAIN} ${is_private} ${batch_files})
+            endif()
+        endforeach()
+    endforeach()
+endfunction()
+
 #############################################################################
 # json_test_should_build_32bit_test(
 #     <build_32bit_var> <build_32bit_only_var> <input>)

docs/mkdocs/docs/api/basic_json/to_bjdata.md

@@ -123,4 +123,6 @@ Linear in the size of the JSON value `j`.
   that is not valid UTF-8 unchanged, as before; `strict` (the default if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.
 - Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
-  array or object was silently skipped, producing invalid BJData.
+  array or object was silently skipped, producing invalid BJData.
+- Writes unsigned integers wider than 64 bits as high-precision numbers since version 3.13.0; previously, they were
+  silently truncated to 64 bits.

docs/mkdocs/docs/api/basic_json/to_bon8.md

@@ -37,8 +37,9 @@ With (2), the bytes written before the exception remain in the output adapter.
 
 ## Exceptions
 
-- Throws [out_of_range.407](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains an unsigned integer
-  above 9223372036854775807, which BON8 cannot represent
+- Throws [out_of_range.407](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains an integer outside
+  the range of int64 (an unsigned integer above 9223372036854775807, or, with a number type wider than 64 bits, any
+  integer beyond int64), which BON8 cannot represent
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if `j` contains a string that is not
   valid UTF-8
 - Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if `j` or a value nested in it is

docs/mkdocs/docs/api/basic_json/to_bson.md

@@ -47,6 +47,10 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 
 - Throws [`type_error.317`](../../home/exceptions.md#jsonexceptiontype_error317) if the top-level type of the JSON value
   is not an object; example: `"to serialize to BSON, top-level type must be object, but is string"`
+- Throws [`out_of_range.407`](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains a signed integer
+  outside the range of int64 or an unsigned integer outside the range of uint64, which is only possible with a number
+  type wider than 64 bits; example:
+  `"integer number 9223372036854775808 cannot be represented by BSON as it does not fit int64"`
 - Throws [`out_of_range.409`](../../home/exceptions.md#jsonexceptionout_of_range409) if a key in the JSON object contains
   a null byte (code point U+0000); example: `"BSON key cannot contain code point U+0000 (at byte 2)"`
 - Throws [`out_of_range.412`](../../home/exceptions.md#jsonexceptionout_of_range412) if the length of a document, array,
@@ -119,3 +123,5 @@ pass before anything is written.
   that is not valid UTF-8 unchanged, as before; `strict` (the default if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316` before anything
   is written.
+- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0; previously, integers of a
+  number type wider than 64 bits were silently truncated.

docs/mkdocs/docs/api/basic_json/to_cbor.md

@@ -46,6 +46,9 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 
 ## Exceptions
 
+- Throws [`out_of_range.407`](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains an integer
+  outside [-2^64, 2^64-1], which is only possible with a number type wider than 64 bits; example:
+  `"integer number 18446744073709551616 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]"`
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if a string or object key in `j` is
   not valid UTF-8 and `error_handler` is `strict` (the default only if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled)
@@ -90,3 +93,5 @@ Linear in the size of the JSON value `j`.
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.
 - Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
   array or object was silently skipped, producing invalid CBOR.
+- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0; previously, integers of a
+  number type wider than 64 bits were silently truncated.

docs/mkdocs/docs/api/basic_json/to_msgpack.md

@@ -46,6 +46,9 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 
 ## Exceptions
 
+- Throws [`out_of_range.407`](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains an integer
+  outside [-2^63, 2^64-1], which is only possible with a number type wider than 64 bits; example:
+  `"integer number 18446744073709551616 cannot be represented by MessagePack as it does not fit [-2^63, 2^64-1]"`
 - Throws [`out_of_range.412`](../../home/exceptions.md#jsonexceptionout_of_range412) if the length of a string, binary
   value, array, or object exceeds 4294967295, the maximum MessagePack can store; example:
   `"MessagePack length 4294967296 exceeds maximum of 4294967295"`
@@ -112,3 +115,5 @@ Linear in the size of the JSON value `j`.
   `number_unsigned_t`.
 - Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
   array or object was silently skipped, producing invalid MessagePack.
+- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0; previously, integers of a
+  number type wider than 64 bits were silently truncated.

docs/mkdocs/docs/api/basic_json_document/index.md

@@ -49,14 +49,21 @@ bookkeeping edits need, and calling any of them on one fails to compile (`#!cpp
 
 - **view_type** - the type of view returned by [`root()`](root.md) (`#!cpp basic_json_view<BasicJsonType, Editable>`)
 - **value_t** - the JSON type enumeration, see [`basic_json::value_t`](../basic_json/value_t.md)
+- **image_check** - how [`load()`](load.md) validates an image (`full`, `bounds`, `none`), see [`image_check`](load.md#image_check)
 
 ## Member functions
 
 - [(constructor)](basic_json_document.md)
+
+### Parsing
+
 - [**parse**](parse.md) (_static_) - deserialize from a compatible input, borrowing or owning it as appropriate
 - [**parse_copy**](parse_copy.md) (_static_) - deserialize a copy of a compatible input
 - [**accept**](accept.md) (_static_) - check whether the input is valid JSON
 - [**read**](read.md) - (re-)parse into this document, reusing its memory
+
+### Access
+
 - [**root**](root.md) - the view of the root value
 - [**is_discarded**](is_discarded.md) - return whether the last parse failed
 - [**source**](source.md) - the parsed text
@@ -64,6 +71,14 @@ bookkeeping edits need, and calling any of them on one fails to compile (`#!cpp
 - [**node_count**](node_count.md) - the number of index entries (values plus object keys)
 - [**memory_usage**](memory_usage.md) - the number of bytes held by the document
 - [**shrink_to_fit**](shrink_to_fit.md) - release unused index capacity
+
+### Images
+
+- [**save**](save.md) - the document as an image that `load()` reads without parsing
+- [**load**](load.md) (_static_) - read an image written by `save()`
+
+### Edits
+
 - [**set**](set.md) - replace a value, or set an object member, an array element, or the value a JSON pointer refers
   to (`#!cpp Editable` documents only)
 - [**push_back**](push_back.md) - append to an array (`#!cpp Editable` documents only)

docs/mkdocs/docs/api/basic_json_document/load.md

@@ -0,0 +1,179 @@
+# <small>nlohmann::basic_json_document::</small>load
+
+```cpp
+// (1)
+static basic_json_document load(const std::uint8_t* image, std::size_t size,
+                                 const image_check check = image_check::full);
+
+// (2)
+static basic_json_document load(const std::vector<std::uint8_t>& image,
+                                 const image_check check = image_check::full);
+
+// (3)
+static basic_json_document load(std::vector<std::uint8_t>&& image,
+                                 const image_check check = image_check::full);
+```
+
+1. Reads an image [`save()`](save.md) wrote, from a pointer and a byte count. The image is **borrowed**: `image`
+   must stay alive and unchanged for as long as the returned document, and any view taken from it, is used.
+2. Reads an image from a `#!cpp std::vector`. Also **borrowed** -- equivalent to overload 1 called with
+   `#!cpp image.data()` and `#!cpp image.size()`.
+3. Reads an image, keeping the vector instead of copying it: `image` is moved into the document (no copy), which
+   then owns it for as long as it needs the text and the decoded strings. [`owns_source()`](owns_source.md) is
+   `#!cpp true` afterward.
+
+In every overload, the node index is copied into storage the document itself owns -- so that it is properly aligned,
+and, for an [editable](index.md#edits) document, can be edited -- while the text and the decoded strings stay in
+`image`. The hash indexes [large objects](../../features/json_view.md) use for lookup are rebuilt, exactly as after
+parsing.
+
+## Parameters
+
+`image` (in)
+:   the image [`save()`](save.md) wrote (overloads 1 and 2), or one to take ownership of (overload 3)
+
+`size` (in)
+:   the number of bytes at `image` (overload 1)
+
+`check` (in)
+:   how thoroughly to validate `image` before trusting it; see [`image_check`](#image_check) below (optional,
+    `#!cpp image_check::full` by default)
+
+## Return value
+
+The document read from the image.
+
+## Exception safety
+
+Overloads 1 and 2 give the strong guarantee: `image` is only read, never written, so a thrown exception leaves the
+caller's buffer untouched.
+
+Overload 3 moves `image` into the document *before* validating it, so that a good image is kept without a copy. If
+loading then fails, the partially built document -- and the vector now inside it -- is discarded along with the
+exception, and `image` itself is left **empty**, not restored to what was passed in. Move a copy in instead, or
+validate with overload 2 first, if the original vector must survive a failed load.
+
+## Exceptions
+
+On a big-endian target, throws [`type_error.320`](../../home/exceptions.md#jsonexceptiontype_error320) -- the same
+exception [`save()`](save.md#exceptions) throws there, since the image format is little-endian only.
+
+Otherwise throws [`parse_error.116`](../../home/exceptions.md#jsonexceptionparse_error116) if `image` is not one
+`save()` could have written, or fails the requested `check`:
+
+| message               | when                                                                                                |
+|------------------------|------------------------------------------------------------------------------------------------------|
+| `too short`            | `image` is `#!cpp nullptr`, or `size` is smaller than the 64-byte header                             |
+| `unknown format`       | the header's magic bytes or version do not match, or a reserved header field is not zero             |
+| `sizes out of range`   | the node count, text size, or decoded-string size the header describes does not fit `size`, or the `#!cpp '\0'` after the text or after the decoded strings is missing |
+| `the check failed`     | `check` is not `#!cpp image_check::none`, and the image fails it -- see [`image_check`](#image_check) |
+
+!!! failure "Example messages"
+
+    ```
+    [json.exception.parse_error.116] parse error: invalid json_document image: too short
+    ```
+    ```
+    [json.exception.parse_error.116] parse error: invalid json_document image: unknown format
+    ```
+    ```
+    [json.exception.parse_error.116] parse error: invalid json_document image: sizes out of range
+    ```
+    ```
+    [json.exception.parse_error.116] parse error: invalid json_document image: the check failed
+    ```
+
+## Complexity
+
+Linear in the number of nodes, which are always copied into the document. `#!cpp image_check::none` does not read
+the text or the decoded strings. `#!cpp image_check::bounds` reads one byte of the text for each float token (its sign,
+to check the recorded digits against the token's length), and nothing else of the text or the decoded strings.
+
+With `#!cpp image_check::full`, linear in the size of the image (the nodes, the text, and the decoded strings), plus
+sorting the ranges of the strings and of the float tokens (at most one per node): the contents of each distinct range
+are checked once, however many nodes refer to it.
+
+## Notes
+
+### `image_check` { #image_check }
+
+```cpp
+using image_check = detail::view::image_check;
+
+enum class image_check
+{
+    full,
+    bounds,
+    none
+};
+```
+
+How thoroughly `load()` validates `image` before trusting it.
+
+`full` (the default)
+:   Checks everything the parser itself guarantees: structure and bounds; that every string is valid UTF-8 (and, for a
+    string still in the source text, that it contains no quote, backslash, or control character); and that every number
+    token is well-formed and matches the value stored for it. Strings and float tokens may share a range only if the
+    ranges are identical (as nodes that share a value do), and never overlap otherwise. Reading and serializing a
+    checked image is safe and always produces valid JSON, exactly as for a parsed document. This is the right choice for
+    an image from anything you do not fully control -- a file, a cache shared with other processes, a peer on the
+    network.
+
+`bounds`
+:   Checks structure and bounds only: that every offset and count in the node index stays inside the image. So
+    `load()`, [`root()`](root.md), element access, and [`materialize()`](../basic_json_view/materialize.md) can never
+    read outside the image, even on a damaged one. It does *not* guarantee that the image describes valid JSON: a
+    string that a `full` check would have rejected can make [`dump()`](../basic_json_view/dump.md) write invalid UTF-8
+    or invalid JSON, or throw [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316), and a number can
+    read back with a value that does not match how it is spelled. An [editable document](../json_editable_document.md)
+    does not take such a string over: [`set`](set.md), [`push_back`](push_back.md) and [`insert`](insert.md) throw
+    `type_error.316` when they copy it from a view of the loaded document, and leave the editable document unchanged.
+    `bounds` skips scanning the text and the decoded strings, so it fits a cache your own process just wrote and reads
+    straight back, where damage would mean a bug or a hardware fault rather than adversarial input.
+
+`none`
+:   Checks nothing. Reading a damaged image is undefined behavior, so use it only for an image you trust as much as
+    your own memory.
+
+**Lifetime.** Overloads 1 and 2 borrow `image`: it must stay alive and byte-for-byte unchanged for as long as the
+returned document, and any [view](../basic_json_view/index.md) taken from it, is used -- exactly like a document
+[`parse()`](parse.md) borrowed its input for. Overload 3 avoids this by keeping the vector itself; see
+[`owns_source`](owns_source.md).
+
+!!! warning "Experimental"
+
+    The image format is versioned but not yet stable, and may change in an incompatible way before it is declared
+    stable; `load()` already rejects an image written by a different format version with `parse_error.116`
+    ("unknown format"). Use images to cache a document within one build of the library, or to hand one to another
+    process running the *same* build on the *same* (little-endian) machine -- not as a long-term storage format.
+
+## Examples
+
+??? example "Caching a document, ownership, and a rejected image"
+
+    The example below saves a parsed document as an image, checks that `load()` reproduces the original
+    [`dump()`](../basic_json_view/dump.md) without parsing, and shows the difference between
+    `load(std::move(image))` (owned) and `load(image)` (borrowed). It then damages one byte of the image and shows
+    `image_check::full` rejecting it with `parse_error.116`, while `image_check::bounds` -- meant for a cache the
+    process already trusts -- still reads it without going out of bounds.
+
+    ```cpp
+    --8<-- "examples/basic_json_document__load.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_document__load.output"
+    ```
+
+## See also
+
+- [save](save.md) - write the document as an image
+- [owns_source](owns_source.md) - return whether the document holds its own copy of the text
+- [parse](parse.md) - deserialize from JSON text instead of an image
+- [Images](../../features/json_view.md#images) - why and when to use images
+
+## Version history
+
+- Added in version 3.13.0.

docs/mkdocs/docs/api/basic_json_document/parse.md

@@ -149,6 +149,7 @@ buffer of a given length, pass a pair of pointers: `#!cpp parse(ptr, ptr + len)`
 - [accept](accept.md) - check whether the input is valid JSON
 - [read](read.md) - (re-)parse into this document, reusing its memory
 - [owns_source](owns_source.md) - return whether the document holds its own copy of the text
+- [load](load.md) - read a document from an image instead of parsing JSON text
 - [`BasicJsonType::parse`](../basic_json/parse.md) - the corresponding function of `basic_json`
 
 ## Version history

docs/mkdocs/docs/api/basic_json_document/read.md

@@ -80,6 +80,7 @@ compile; see [`parse`](parse.md#notes).
 
 - [parse](parse.md) - deserialize from a compatible input
 - [root](root.md) - the view of the root value
+- [load](load.md) - read a document from an image instead of parsing JSON text
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json_document/save.md

@@ -0,0 +1,104 @@
+# <small>nlohmann::basic_json_document::</small>save
+
+```cpp
+std::vector<std::uint8_t> save() const;
+```
+
+Writes the document as an *image*: a byte buffer that [`load`](load.md) reads back without parsing. The image holds
+the node index, the source text (plus, for an edited document, the number tokens edits wrote), and the decoded
+strings (plus the strings edits wrote) -- everything [`root()`](root.md) needs, with nothing left to parse.
+
+An edited document is written in its *current* state, with its values in document order, the way the library's own
+parser would have produced them for that JSON text: a member [`set`](set.md) added goes at the end, an
+[`erase`](erase.md)d member leaves no trace, and a float that is not finite (NaN or positive/negative infinity)
+becomes null, the same substitution [`dump()`](../basic_json_view/dump.md) makes. The same document always saves to
+the same bytes -- also across `BasicJsonType` and `#!cpp Editable`, since the image reflects document order and
+values only, not which specialization produced them.
+
+## Return value
+
+The image, as a `#!cpp std::vector<std::uint8_t>`. Pass it, or a pointer to its data together with its size, to
+[`load`](load.md) to read the document back.
+
+## Exception safety
+
+Strong guarantee: `save()` does not modify `#!cpp *this` (it is `#!cpp const`), so if it throws, the document is left
+exactly as it was, and the partially built image is discarded with the exception.
+
+## Exceptions
+
+Throws [`type_error.320`](../../home/exceptions.md#jsonexceptiontype_error320) if the document is
+[discarded](is_discarded.md) -- a default-constructed document, or one a failed [`parse()`](parse.md)/
+[`read()`](read.md) with `allow_exceptions == false` left discarded.
+
+On a big-endian target, throws `type_error.320` with a different message instead: the image format is little-endian
+only (see [Notes](#notes)).
+
+Throws [`out_of_range.416`](../../home/exceptions.md#jsonexceptionout_of_range416) if the node count, the text, or
+the decoded strings of the image would individually reach 4 GiB -- the same 32-bit offsets
+[`parse()`](parse.md#exceptions) and, for edits, [`set`](set.md)/[`push_back`](push_back.md) are already limited to.
+
+!!! failure "Example messages"
+
+    ```
+    [json.exception.type_error.320] cannot save a discarded json_document
+    ```
+    ```
+    [json.exception.type_error.320] json_document images need a little-endian target
+    ```
+    ```
+    [json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document
+    ```
+
+## Complexity
+
+Linear in the size of the document: the number of nodes, plus the length of the text and the decoded strings that end
+up in the image.
+
+## Notes
+
+**Format.** The image begins with a 64-byte header (the magic bytes `#!cpp "NJVI"`, a version number, the node count,
+and the sizes of the text and the decoded strings, all little-endian), followed by the nodes
+([16 bytes each](../../home/architecture.md#node-index-of-json-views)), the text and a `#!cpp '\0'`, and the decoded
+strings and a `#!cpp '\0'`. [`load`](load.md) checks the header, and the sizes it describes, before reading anything
+else -- see [`load`'s Exceptions](load.md#exceptions).
+
+!!! warning "Experimental"
+
+    The image format is versioned but not yet stable: it may change in an incompatible way before it is declared
+    stable. Use images to cache a document within one build of the library, or to hand one to another process running
+    the *same* build on the *same* (little-endian) machine -- not as a long-term storage format. Keep the original
+    JSON text if you need to read a saved document back with a future library version.
+
+**Little-endian only.** The image is written as raw little-endian bytes, with no byte-swapping. `save()` (and
+[`load`](load.md)) throw `type_error.320` on a big-endian target rather than silently produce bytes a big-endian
+reader could not interpret correctly.
+
+## Examples
+
+??? example "Caching a parsed document as an image"
+
+    The example below saves a parsed configuration as an image -- the way a service might cache one to answer later
+    requests without parsing the text again -- and confirms that loading it back gives exactly the same result as
+    parsing did, and that saving is deterministic.
+
+    ```cpp
+    --8<-- "examples/basic_json_document__save.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_document__save.output"
+    ```
+
+## See also
+
+- [load](load.md) - read an image written by `save()`
+- [owns_source](owns_source.md) - return whether the document holds its own copy of the text
+- [`basic_json_view::dump`](../basic_json_view/dump.md) - serialize the document to JSON text instead of an image
+- [Images](../../features/json_view.md#images) - why and when to use images
+
+## Version history
+
+- Added in version 3.13.0.

docs/mkdocs/docs/api/macros/json_disable_unlocked_file_io.md

@@ -10,6 +10,9 @@ earlier, instead of locking the file once and reading it with the unlocked stdio
 It does not change any result: values, exceptions, and the position of the file after parsing are the same either way.
 Only the speed and the locking of the file differ (see the notes on locking below).
 
+Builds with MemorySanitizer use `std::fgetc` automatically, because the inline `getc_unlocked` of glibc reads the stdio
+buffer directly, which glibc (not instrumented) filled, and MemorySanitizer would report it as uninitialized.
+
 ## Default definition
 
 The default value is `0`: the file is locked once and read with the unlocked stdio functions where they are available.

docs/mkdocs/docs/examples/basic_json_document__load.cpp

@@ -0,0 +1,52 @@
+#include <cstdint>
+#include <iostream>
+#include <string>
+#include <vector>
+#include <nlohmann/json_view.hpp>
+
+using json = nlohmann::json;
+using json_document = nlohmann::json_document;
+using image_check = json_document::image_check;
+
+int main()
+{
+    std::cout << std::boolalpha;
+
+    // the image of a parsed document -- as if read back from a cache file or
+    // received from another process running the same build of the library
+    const std::string text = R"({"name": "cache", "note": "caf\u00e9", "replicas": ["db2", "db3"]})";
+    const json_document parsed = json_document::parse(text);
+    const std::vector<std::uint8_t> image = parsed.save();
+
+    // (1)/(2) load() needs no parsing, yet dumps exactly what parsing did
+    const json_document borrowed = json_document::load(image);
+    std::cout << (borrowed.root().dump() == parsed.root().dump()) << '\n';
+    std::cout << borrowed.owns_source() << '\n'; // borrowed: still points into `image`
+
+    // (3) load(std::move(image)) keeps the vector instead of copying it
+    std::vector<std::uint8_t> to_move = image;
+    const json_document owned = json_document::load(std::move(to_move));
+    std::cout << owned.owns_source() << '\n';
+
+    // a damaged image -- the last byte of the decoded string "note" holds
+    // (an escape sequence, so it was unescaped into the document's own
+    // buffer), flipped, as storage or transport corruption might do
+    std::vector<std::uint8_t> damaged = image;
+    damaged[damaged.size() - 2] = 0xFF;
+
+    // image_check::full inspects strings and numbers, so it catches the damage
+    try
+    {
+        static_cast<void>(json_document::load(damaged, image_check::full));
+    }
+    catch (const json::parse_error& e)
+    {
+        std::cout << e.id << '\n';
+    }
+
+    // image_check::bounds only checks structure and bounds, so a cache the
+    // process already trusts loads without the extra scan -- reading a value
+    // the damage did not touch is still safe
+    const json_document trusted = json_document::load(damaged, image_check::bounds);
+    std::cout << trusted.root()["name"].get<std::string>() << '\n';
+}

docs/mkdocs/docs/examples/basic_json_document__load.output

@@ -0,0 +1,5 @@
+true
+false
+true
+116
+cache

docs/mkdocs/docs/examples/basic_json_document__save.cpp

@@ -0,0 +1,30 @@
+#include <cstdint>
+#include <iostream>
+#include <string>
+#include <vector>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    std::cout << std::boolalpha;
+
+    // a configuration a service parses once and then caches as an image, so
+    // that later requests can load() it instead of parsing the text again
+    const std::string text = R"({"name": "cache", "host": "db1", "port": 6379, "replicas": ["db2", "db3"]})";
+    const json_document config = json_document::parse(text);
+
+    // save() turns the parsed document into a byte buffer: a 64-byte header,
+    // the node index, the source text, and the decoded strings
+    const std::vector<std::uint8_t> image = config.save();
+    std::cout << image.size() << '\n';
+
+    // the same document always saves to the same bytes
+    std::cout << (image == json_document::parse(text).save()) << '\n';
+
+    // loading the image back needs no parsing, yet dumps exactly what
+    // parsing the text produced
+    const json_document reloaded = json_document::load(image);
+    std::cout << (reloaded.root().dump() == config.root().dump()) << '\n';
+}

docs/mkdocs/docs/examples/basic_json_document__save.output

@@ -0,0 +1,3 @@
+316
+true
+true

docs/mkdocs/docs/features/index.md

@@ -14,7 +14,8 @@ C++ types, and finally serialize it again.
   [parsing untrusted input](parsing/untrusted_input.md).
 - [Zero-copy JSON views](json_view.md) — read a JSON text through a flat index instead of building a `json` tree;
   strings and numbers stay in the input and are only decoded when needed.
-  [Editable documents](json_view.md#editing-a-document) can also be modified.
+- [Editable documents](json_view.md#editing-a-document) — modify a parsed document without building a `json` tree.
+- [Images](json_view.md#images) — save a parsed document and load it again without parsing it.
 - [Comments](comments.md) and [trailing commas](trailing_commas.md) — opt-in relaxations of the JSON grammar.
 
 ## Accessing and modifying values

docs/mkdocs/docs/features/json_view.md

@@ -312,6 +312,57 @@ edits. See [`basic_json_document`'s Edits](../api/basic_json_document/index.md#e
 throws (the *basic* guarantee, not the strong one `dump()` and the read-only functions provide). How edits are kept in
 the index is described in the [architecture overview](../home/architecture.md#node-index-of-json-views).
 
+## Images
+
+[`save()`](../api/basic_json_document/save.md) writes a document as an *image*: a byte buffer that
+[`load()`](../api/basic_json_document/load.md) reads back into a document without parsing -- no lexing, no building
+the node index, nothing but copying the nodes and pointing the text and the decoded strings at the image. Where
+[`parse_copy()`](../api/basic_json_document/parse_copy.md) still has to scan the whole input,
+[`load()`](../api/basic_json_document/load.md) turns that scan into a copy of the node index alone.
+
+**Why.** A document that is parsed once and then read many times -- a configuration loaded at startup, a template
+rendered on every request, a large reference dataset a worker process needs in memory -- pays for parsing once but
+can amortize [`save()`](../api/basic_json_document/save.md)'s cost across every later load. That makes images useful
+for a cache: save a document the first time it is parsed (to a file, a shared-memory segment, an in-process cache),
+and [`load()`](../api/basic_json_document/load.md) it on every later use instead of parsing the source text again.
+They are just as useful for handing a parsed document to another process (or a forked worker) running the same build
+of the library, since [`load()`](../api/basic_json_document/load.md) turns the transfer into a copy of the node index
+plus pointers into the received bytes, not a re-parse.
+
+**Choosing a check.** [`load()`](../api/basic_json_document/load.md) takes an
+[`image_check`](../api/basic_json_document/load.md#image_check) that trades validation against speed:
+`image_check::full` (the default) checks everything the parser itself guarantees, so a checked image is exactly as
+safe to read and serialize as a freshly parsed document -- the right choice whenever the image did not come straight
+from this process's own [`save()`](../api/basic_json_document/save.md), such as a file or a network peer.
+`image_check::bounds` only checks structure and bounds -- cheaper, since it skips scanning the text and the decoded
+strings -- and fits a cache the process trusts, one it wrote and reads back itself. `image_check::none` skips
+validation entirely, for an image trusted as much as the process's own memory. See
+[`load()`'s Notes](../api/basic_json_document/load.md#notes) for exactly what each level does and does not guarantee.
+
+??? example "Example: cache a parsed configuration as an image"
+
+    ```cpp
+    --8<-- "examples/basic_json_document__save.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_document__save.output"
+    ```
+
+!!! warning "Experimental"
+
+    The image format is versioned but not yet stable, and may change in an incompatible way before it is declared
+    stable. It is little-endian only, and tied to the library build that wrote it -- use it to cache a document or to
+    hand one to another process running the *same* build, not as a long-term storage format; keep the original JSON
+    text if a saved document needs to be readable by a future library version.
+
+The idea of a document you can read without parsing comes from zero-copy formats such as
+[FlatBuffers](https://github.com/google/flatbuffers) and [YaFF](https://github.com/yandex/yaff); the check
+[`load()`](../api/basic_json_document/load.md) runs follows the idea of FlatBuffers' Verifier. No code is taken from
+either.
+
 ## Choosing between `json`, `ordered_json`, the SAX interface, and `json_view`
 
 | | [`json`](../api/json.md) / [`ordered_json`](../api/ordered_json.md) | [SAX interface](parsing/sax_interface.md) | [`json_document`](../api/json_document.md) / [`json_view`](../api/json_view.md) | [`json_editable_document`](../api/json_editable_document.md) / [`json_editable_view`](../api/json_editable_view.md) |
@@ -320,6 +371,7 @@ the index is described in the [architecture overview](../home/architecture.md#no
 | **Mutability** | freely mutable | not applicable (a one-shot event stream) | read-only | [`set`](../api/basic_json_document/set.md)/[`push_back`](../api/basic_json_document/push_back.md)/[`insert`](../api/basic_json_document/insert.md)/[`erase`](../api/basic_json_document/erase.md) edit in place; the source text is never rewritten |
 | **What you get** | a full tree you can read, write, and keep as long as you like | a sequence of callbacks; whatever your handler builds from them | a flat index plus, on demand, [`materialize()`](../api/basic_json_view/materialize.md)d `json`/`ordered_json` values for the parts you actually use | the same, plus [`dump()`](../api/basic_json_view/dump.md) of an edited document that keeps the member order and, with [`number_format::source`](../api/basic_json_view/number_format.md), the spelling of every untouched number |
 | **Typical use** | general-purpose JSON handling: config, request/response bodies you build or modify, anything you hold onto | validating or projecting a text into your own data structure without ever holding the whole thing as JSON | large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives | a document you read, patch a few fields of, and write back -- a configuration file, for instance -- where the rest of it should come back exactly as it was |
+| **Caching/reload** | not applicable -- re-parse, or roll your own serialization | not applicable | [`save()`](../api/basic_json_document/save.md)/[`load()`](../api/basic_json_document/load.md): cache the parsed index as an image and reload it without parsing | same, saving the document's current -- possibly edited -- state |
 
 ## Version history
 

docs/mkdocs/docs/home/architecture.md

@@ -262,6 +262,14 @@ edited:
 - Views of read-only documents compile without any of this: how views walk the index is a template parameter
   (`navigation<Editable>`).
 
+Images ([`save`](../api/basic_json_document/save.md) and [`load`](../api/basic_json_document/load.md),
+[`detail/view/image.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/image.hpp)) store
+the nodes as they are: a 64-byte header (the magic bytes `NJVI`, a format version, the sizes, and reserved bytes that
+must be zero), the nodes, the text, and the decoded strings. An edited document is first written in document order, as
+the parser would have written it (without links), and the numbers of the hash indexes are cleared, since `load`
+rebuilds the indexes. So a change of the node layout is a change of the image format: it must raise `image_version`,
+and `load` then rejects images of other versions (`parse_error.116`) instead of misreading them.
+
 ## Input adapters
 
 Input is read via **input adapters** that abstract a source. Every input adapter provides this interface:

docs/mkdocs/docs/home/exceptions.md

@@ -395,6 +395,23 @@ A UBJSON high-precision number could not be parsed.
     [json.exception.parse_error.115] parse error at byte 5: syntax error while parsing UBJSON high-precision number: invalid number text: 1A
     ```
 
+### json.exception.parse_error.116
+
+[`basic_json_document::load()`](../api/basic_json_document/load.md) rejected an
+[image](../features/json_view.md#images): either the bytes are not one [`save()`](../api/basic_json_document/save.md)
+could have written (too short, an unknown magic number or format version, or sizes that do not fit the buffer), or
+they are, but fail the requested [`image_check`](../api/basic_json_document/load.md#image_check).
+
+!!! failure "Example message"
+
+    ```
+    [json.exception.parse_error.116] parse error: invalid json_document image: the check failed
+    ```
+
+!!! note
+
+    This exception was added in version 3.13.0, together with [images](../features/json_view.md#images).
+
 ## Iterator errors
 
 This exception is thrown if iterators passed to a library function do not match
@@ -831,6 +848,26 @@ from JSON text.
 
     This exception was added in version 3.13.0, together with editable [`json_document`s](../features/json_view.md).
 
+### json.exception.type_error.320
+
+[`basic_json_document::save()`](../api/basic_json_document/save.md) cannot write an
+[image](../features/json_view.md#images) of a [discarded](../api/basic_json_document/is_discarded.md) document.
+[`save()`](../api/basic_json_document/save.md) and [`load()`](../api/basic_json_document/load.md) also throw this
+exception on a big-endian target, since the image format is little-endian only.
+
+!!! failure "Example messages"
+
+    ```
+    [json.exception.type_error.320] cannot save a discarded json_document
+    ```
+    ```
+    [json.exception.type_error.320] json_document images need a little-endian target
+    ```
+
+!!! note
+
+    This exception was added in version 3.13.0, together with [images](../features/json_view.md#images).
+
 ### json.exception.type_error.321
 
 A discarded value (one created by [`parse()`](../api/basic_json/parse.md) with a callback that returns `false` for the
@@ -935,9 +972,19 @@ double-precision number when `number_float_t` is `#!cpp float`.
 
 ### json.exception.out_of_range.407
 
-An unsigned integer number greater than 9223372036854775807 cannot be represented in the binary format. This exception
-is only thrown by [`to_bon8`](../api/basic_json/to_bon8.md), as [BON8](../features/binary_formats/bon8.md) has no
-unsigned 64-bit integer type.
+An integer number cannot be represented by the binary format it is serialized to:
+
+- [BON8](../features/binary_formats/bon8.md) only stores integers that fit into int64, so
+  [`to_bon8`](../api/basic_json/to_bon8.md) throws for unsigned integers greater than 9223372036854775807.
+- [CBOR](../features/binary_formats/cbor.md), [MessagePack](../features/binary_formats/messagepack.md), and
+  [BSON](../features/binary_formats/bson.md) store integers in at most 64 bits. With the default number types, every
+  integer fits, but a [`number_integer_t`](../api/basic_json/number_integer_t.md) or
+  [`number_unsigned_t`](../api/basic_json/number_unsigned_t.md) wider than 64 bits (e.g., `__int128`) can hold values
+  outside the range of the format: [-2^64, 2^64-1] for CBOR, [-2^63, 2^64-1] for MessagePack, and the range of int64
+  (signed integers) or uint64 (unsigned integers) for BSON.
+
+[UBJSON](../features/binary_formats/ubjson.md) and [BJData](../features/binary_formats/bjdata.md) never throw this
+exception, because they serialize integers beyond 64 bits as high-precision numbers.
 
 !!! note "Former occurrences"
 
@@ -946,11 +993,17 @@ unsigned 64-bit integer type.
     - Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers.
     - Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers.
 
-!!! failure "Example message"
+    Before version 3.13.0, CBOR, MessagePack, and BSON silently truncated integers wider than 64 bits, and BJData
+    truncated unsigned integers wider than 64 bits.
+
+!!! failure "Example messages"
 
     ```
     integer number 18446744073709551615 cannot be represented by BON8 as it does not fit int64
     ```
+    ```
+    integer number 1267650600228229401496703205376 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]
+    ```
 
 ### json.exception.out_of_range.408
 
@@ -1088,7 +1141,9 @@ MessagePack's ext type and BSON's binary subtype are each stored in a single byt
 so they do not support an input of 4294967280 bytes (4 GiB minus 16 bytes) or more. The same 32-bit limit applies to an **editable** document's own
 storage: [`set`](../api/basic_json_document/set.md) and [`push_back`](../api/basic_json_document/push_back.md) throw
 this exception once the strings and number tokens written by edits reach 4 GiB in total, or once more than
-4294967295 arrays/objects have had an element set or appended to them.
+4294967295 arrays/objects have had an element set or appended to them. The same limit applies to an
+[image](../features/json_view.md#images): [`save()`](../api/basic_json_document/save.md) throws it if the node
+count, the text, or the decoded strings it would write would individually reach 4 GiB.
 
 !!! failure "Example messages"
 
@@ -1098,6 +1153,9 @@ this exception once the strings and number tokens written by edits reach 4 GiB i
     ```
     [json.exception.out_of_range.416] edits of 4 GiB or more are not supported by json_document
     ```
+    ```
+    [json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document
+    ```
 
 !!! note
 

docs/mkdocs/docs/integration/cmake.md

@@ -235,6 +235,17 @@ Build the unit tests against the [simdutf](https://github.com/simdutf/simdutf) U
 its version is set by the cache variable `JSON_SIMDUTF_VERSION`. This option is `OFF` by default. Depends on
 `JSON_BuildTests`.
 
+### `JSON_TestUnityBuild`
+
+Build the unit tests in batches of several test files per executable to speed up compilation: the files of a batch are
+compiled as one translation unit, so the template instantiations of the library are shared. Every test file still gets
+its own CTest test (e.g., `test-foo_cpp11`), which runs only the test cases of that file from the shared executable.
+This option is `ON` by default (and `OFF` with MinGW). Set it to `OFF` to get one executable per test file, e.g., when
+debugging a single file. The number of files per executable is set by the cache variable `JSON_TestUnityBatchSize`
+(default: `8`). Related test files (e.g., all binary formats) are grouped explicitly in `tests/CMakeLists.txt`; each
+group is compiled as one executable per C++ standard regardless of the batch size, and only the remaining files are
+batched by size. Depends on `JSON_BuildTests`.
+
 ### `JSON_Valgrind`
 
 Execute the test suite with [Valgrind](https://valgrind.org). This option is `OFF` by default. Depends on `JSON_BuildTests`.

docs/mkdocs/mkdocs.yml

@@ -242,6 +242,7 @@ nav:
           - 'erase': api/basic_json_document/erase.md
           - 'insert': api/basic_json_document/insert.md
           - 'is_discarded': api/basic_json_document/is_discarded.md
+          - 'load': api/basic_json_document/load.md
           - 'memory_usage': api/basic_json_document/memory_usage.md
           - 'node_count': api/basic_json_document/node_count.md
           - 'owns_source': api/basic_json_document/owns_source.md
@@ -250,6 +251,7 @@ nav:
           - 'push_back': api/basic_json_document/push_back.md
           - 'read': api/basic_json_document/read.md
           - 'root': api/basic_json_document/root.md
+          - 'save': api/basic_json_document/save.md
           - 'set': api/basic_json_document/set.md
           - 'shrink_to_fit': api/basic_json_document/shrink_to_fit.md
           - 'source': api/basic_json_document/source.md

include/nlohmann/detail/conversions/to_chars.hpp

@@ -1342,6 +1342,66 @@ inline char* write_shortest(char* first, const zmij::shortest_decimal d) noexcep
     return end + (three ? 5 : 4);
 }
 
+/// the powers of ten up to 10^19 (all that fit in std::uint64_t)
+inline const std::array<std::uint64_t, 20>& powers_of_ten() noexcept
+{
+    static const std::array<std::uint64_t, 20> powers =
+    {
+        {
+            1u,
+            10u,
+            100u,
+            1000u,
+            10000u,
+            100000u,
+            1000000u,
+            10000000u,
+            100000000u,
+            1000000000u,
+            10000000000u,
+            100000000000u,
+            1000000000000u,
+            10000000000000u,
+            100000000000000u,
+            1000000000000000u,
+            10000000000000000u,
+            100000000000000000u,
+            1000000000000000000u,
+            10000000000000000000u
+        }
+    };
+    return powers;
+}
+
+/*!
+@brief digits * 10^exp, as write_decimal() writes it, for the digits of a
+double that need no conversion (count digits, at most 15, the first not 0;
+trailing zeros allowed): extended to 16 digits and written by write_shortest()
+
+@return a pointer past the text; up to 41 bytes at @a first are written
+        (some beyond the returned end)
+*/
+JSON_HEDLEY_NON_NULL(1)
+JSON_HEDLEY_RETURNS_NON_NULL
+inline char* write_short_decimal(char* first, std::uint64_t digits, int count, int exp) noexcept
+{
+    JSON_ASSERT(digits >= powers_of_ten()[static_cast<std::size_t>(count - 1)] && count <= 15);
+    const int scale = 16 - count;
+    return write_shortest(first, zmij::shortest_decimal{digits * powers_of_ten()[static_cast<std::size_t>(scale)], exp - scale - 1, 0, false});
+}
+
+/// as write_short_decimal(), counting the digits (not 0, less than 10^15)
+JSON_HEDLEY_NON_NULL(1)
+JSON_HEDLEY_RETURNS_NON_NULL
+inline char* write_short_decimal(char* first, std::uint64_t digits, int exp) noexcept
+{
+    JSON_ASSERT(digits != 0 && digits < 1000000000000000u);
+    // floor(log10(2^bits)) + 1 digits, or one less
+    const int log2_bound = ((64 - count_leading_zeros(digits)) * 1233) >> 12;
+    const int count = log2_bound + (digits >= powers_of_ten()[static_cast<std::size_t>(log2_bound)] ? 1 : 0);
+    return write_short_decimal(first, digits, count, exp);
+}
+
 /*!
 @brief whether FloatType is an IEEE 754 binary64 type (a double, or a long double
 that has the same format, as with MSVC and on Apple's Arm CPUs)

include/nlohmann/detail/input/input_adapters.hpp

@@ -38,8 +38,16 @@
     // 2 = Microsoft CRT _lock_file/_fgetc_nolock, 0 = none (std::fgetc, which
     // locks the file for every character). Only C libraries known to declare
     // the functions are listed; the macros are tested after <cstdio>, which
-    // defines __GLIBC__.
-    #if JSON_DISABLE_UNLOCKED_FILE_IO
+    // defines __GLIBC__. MemorySanitizer builds always use std::fgetc: glibc's
+    // inline getc_unlocked reads the stdio buffer directly, which glibc (not
+    // instrumented) filled, so MemorySanitizer would report a false positive.
+    #if defined(__has_feature)
+        #if __has_feature(memory_sanitizer)
+            #define JSON_UNLOCKED_FILE_IO_MSAN 1
+        #endif
+    #endif
+
+    #if JSON_DISABLE_UNLOCKED_FILE_IO || defined(JSON_UNLOCKED_FILE_IO_MSAN)
         #define JSON_UNLOCKED_FILE_IO 0
     #elif defined(_MSC_VER)
         #define JSON_UNLOCKED_FILE_IO 2
@@ -48,6 +56,7 @@
     #else
         #define JSON_UNLOCKED_FILE_IO 0
     #endif
+    #undef JSON_UNLOCKED_FILE_IO_MSAN
 #endif  // JSON_NO_IO
 
 NLOHMANN_JSON_NAMESPACE_BEGIN

include/nlohmann/detail/macro_unscope.hpp

@@ -19,8 +19,6 @@
 #undef NLOHMANN_CAN_CALL_STD_FUNC_IMPL
 #undef JSON_INLINE_VARIABLE
 #undef JSON_NO_UNIQUE_ADDRESS
-#undef JSON_DISABLE_ENUM_SERIALIZATION
-#undef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
 #undef JSON_UNLOCKED_FILE_IO
 #undef JSON_DTOA_SSE2
 #undef JSON_DTOA_NEON
@@ -48,6 +46,8 @@
     #undef JSON_STRICT_BINARY_UTF8
     #undef JSON_DELETE_DEPRECATED_FUNCTIONS
     #undef JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
+    #undef JSON_DISABLE_ENUM_SERIALIZATION
+    #undef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
     #undef JSON_DISABLE_STREAM_BULK_SCAN
     #undef JSON_DISABLE_UNLOCKED_FILE_IO
 #endif

include/nlohmann/detail/output/binary_writer.hpp

@@ -23,6 +23,7 @@
     #include <cstdlib> // _byteswap_ushort, _byteswap_ulong, _byteswap_uint64
 #endif
 
+#include <nlohmann/detail/bit_ops.hpp>
 #include <nlohmann/detail/input/binary_reader.hpp>
 #include <nlohmann/detail/input/string_scan.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
@@ -204,24 +205,12 @@ class binary_writer
             }
 
             case value_t::number_integer:
-            {
-                if (j.m_data.m_value.number_integer >= 0)
-                {
-                    // CBOR does not differentiate between positive signed
-                    // integers and unsigned integers
-                    write_cbor_head(0x00, static_cast<std::uint64_t>(j.m_data.m_value.number_integer));
-                }
-                else
-                {
-                    // a negative integer n is encoded as -1 - n
-                    write_cbor_head(0x20, static_cast<std::uint64_t>(-1 - j.m_data.m_value.number_integer));
-                }
-                break;
-            }
-
             case value_t::number_unsigned:
             {
-                write_cbor_head(0x00, conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+                // one case for both, so that an array mixing negative
+                // (number_integer) and non-negative (number_unsigned) values
+                // does not mispredict the switch
+                write_cbor_integer(j, std::integral_constant<bool, integers_fit_64_bits()>());
                 break;
             }
 
@@ -256,13 +245,7 @@ class binary_writer
 
             case value_t::array:
             {
-                // step 1: write control byte and the array size
-                write_cbor_head(0x80, j.m_data.m_value.array->size());
-
-                for (const auto& el : *j.m_data.m_value.array)
-                {
-                    write_cbor(el, depth + 1);
-                }
+                write_cbor_array(j, depth);
                 break;
             }
 
@@ -420,54 +403,12 @@ class binary_writer
             }
 
             case value_t::number_integer:
-            {
-                if (j.m_data.m_value.number_integer >= 0)
-                {
-                    // MessagePack does not differentiate between positive
-                    // signed integers and unsigned integers.
-                    write_msgpack_unsigned(static_cast<std::uint64_t>(j.m_data.m_value.number_integer));
-                }
-                else
-                {
-                    if (j.m_data.m_value.number_integer >= -32)
-                    {
-                        // negative fixnum
-                        write_number(static_cast<std::int8_t>(j.m_data.m_value.number_integer));
-                    }
-                    else if (j.m_data.m_value.number_integer >= (std::numeric_limits<std::int8_t>::min)() &&
-                             j.m_data.m_value.number_integer <= (std::numeric_limits<std::int8_t>::max)())
-                    {
-                        // int 8
-                        oa.write_character(to_char_type(0xD0));
-                        write_number(static_cast<std::int8_t>(j.m_data.m_value.number_integer));
-                    }
-                    else if (j.m_data.m_value.number_integer >= (std::numeric_limits<std::int16_t>::min)() &&
-                             j.m_data.m_value.number_integer <= (std::numeric_limits<std::int16_t>::max)())
-                    {
-                        // int 16
-                        oa.write_character(to_char_type(0xD1));
-                        write_number(static_cast<std::int16_t>(j.m_data.m_value.number_integer));
-                    }
-                    else if (j.m_data.m_value.number_integer >= (std::numeric_limits<std::int32_t>::min)() &&
-                             j.m_data.m_value.number_integer <= (std::numeric_limits<std::int32_t>::max)())
-                    {
-                        // int 32
-                        oa.write_character(to_char_type(0xD2));
-                        write_number(static_cast<std::int32_t>(j.m_data.m_value.number_integer));
-                    }
-                    else
-                    {
-                        // int 64
-                        oa.write_character(to_char_type(0xD3));
-                        write_number(static_cast<std::int64_t>(j.m_data.m_value.number_integer));
-                    }
-                }
-                break;
-            }
-
             case value_t::number_unsigned:
             {
-                write_msgpack_unsigned(static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+                // one case for both, so that an array mixing negative
+                // (number_integer) and non-negative (number_unsigned) values
+                // does not mispredict the switch
+                write_msgpack_integer(j, std::integral_constant<bool, integers_fit_64_bits()>());
                 break;
             }
 
@@ -485,13 +426,7 @@ class binary_writer
 
             case value_t::array:
             {
-                // step 1: write control byte and the array size
-                write_msgpack_array_prefix(j.m_data.m_value.array->size(), j);
-
-                for (const auto& el : *j.m_data.m_value.array)
-                {
-                    write_msgpack(el, depth + 1);
-                }
+                write_msgpack_array(j, depth);
                 break;
             }
 
@@ -824,6 +759,142 @@ class binary_writer
         JSON_THROW(type_error::create(exception_id::discarded_value_used, concat("cannot serialize discarded value to ", format_name), &j));
     }
 
+    /// how the range of integer type @a NumberType relates to that of @a TargetType
+    template<typename TargetType, typename NumberType>
+    struct integer_range
+    {
+        /// whether NumberType can hold values above the maximum of TargetType
+        static constexpr bool may_exceed_max = std::numeric_limits<NumberType>::digits > std::numeric_limits<TargetType>::digits;
+        /// whether NumberType can hold values below the minimum of TargetType
+        static constexpr bool may_fall_below_min = std::numeric_limits<NumberType>::is_signed
+            && (!std::numeric_limits<TargetType>::is_signed || may_exceed_max);
+    };
+
+    /*!
+    @brief whether integer @a n does not exceed the maximum of @a TargetType
+
+    The binary formats store integers in at most 64 bits, but number_integer_t
+    and number_unsigned_t may be wider (e.g., __int128). The range of the
+    integer is taken from std::numeric_limits, so a number type whose range
+    fits into @a TargetType is never checked: the function is then constant
+    true, and the default int64_t/uint64_t types pay nothing.
+    */
+    template<typename TargetType, typename NumberType>
+    static constexpr bool integer_fits_max(const NumberType n) noexcept
+    {
+        return integer_fits_max<TargetType>(n, std::integral_constant<bool, integer_range<TargetType, NumberType>::may_exceed_max>());
+    }
+
+    template<typename TargetType, typename NumberType>
+    static constexpr bool integer_fits_max(const NumberType /*unused*/, std::false_type /*may_exceed*/) noexcept
+    {
+        return true;
+    }
+
+    template<typename TargetType, typename NumberType>
+    static constexpr bool integer_fits_max(const NumberType n, std::true_type /*may_exceed*/) noexcept
+    {
+        // NumberType has more digits than TargetType, so it can hold its maximum
+        return n <= static_cast<NumberType>((std::numeric_limits<TargetType>::max)());
+    }
+
+    /*!
+    @brief whether integer @a n is not below the minimum of @a TargetType;
+           constant true if NumberType cannot hold such a value
+    */
+    template<typename TargetType, typename NumberType>
+    static constexpr bool integer_fits_min(const NumberType n) noexcept
+    {
+        return integer_fits_min<TargetType>(n, std::integral_constant<bool, integer_range<TargetType, NumberType>::may_fall_below_min>());
+    }
+
+    template<typename TargetType, typename NumberType>
+    static constexpr bool integer_fits_min(const NumberType /*unused*/, std::false_type /*may_fall_below*/) noexcept
+    {
+        return true;
+    }
+
+    template<typename TargetType, typename NumberType>
+    static constexpr bool integer_fits_min(const NumberType n, std::true_type /*may_fall_below*/) noexcept
+    {
+        // NumberType is signed and either TargetType is unsigned (minimum 0)
+        // or NumberType has more digits, so it can hold the minimum
+        return n >= static_cast<NumberType>((std::numeric_limits<TargetType>::min)());
+    }
+
+    /*!
+    @brief whether integer @a n lies in the range of @a TargetType
+    */
+    template<typename TargetType, typename NumberType>
+    static constexpr bool integer_fits(const NumberType n) noexcept
+    {
+        return integer_fits_min<TargetType>(n) && integer_fits_max<TargetType>(n);
+    }
+
+    /*!
+    @brief the decimal digits of the non-negative integer @a n
+    */
+    template<typename NumberType>
+    static std::string integer_to_string(NumberType n, std::false_type /*is_signed*/)
+    {
+        std::string result;
+        do
+        {
+            result.push_back(static_cast<char>('0' + (n % 10)));
+            n = static_cast<NumberType>(n / 10);
+        }
+        while (n != 0);
+        std::reverse(result.begin(), result.end());
+        return result;
+    }
+
+    /*!
+    @brief the decimal digits of the integer @a n, with a leading '-' if it
+           is negative
+    */
+    template<typename NumberType>
+    static std::string integer_to_string(NumberType n, std::true_type /*is_signed*/)
+    {
+        // the remainder of a negative n is not positive; each digit is
+        // negated on its own, because negating the minimum of NumberType
+        // as a whole would overflow
+        const bool negative = n < 0;
+        std::string result;
+        do
+        {
+            const auto remainder = n % 10;
+            result.push_back(static_cast<char>('0' + (remainder < 0 ? -remainder : remainder)));
+            n = static_cast<NumberType>(n / 10);
+        }
+        while (n != 0);
+        if (negative)
+        {
+            result.push_back('-');
+        }
+        std::reverse(result.begin(), result.end());
+        return result;
+    }
+
+    /*!
+    @brief throws because the integer @a j is too large for @a format_name
+    @throw out_of_range.407 always
+    */
+    JSON_HEDLEY_NO_RETURN static void throw_integer_out_of_range(const BasicJsonType& j, const char* format_name, const char* range)
+    {
+        // neither std::to_string(), which has no overload for integer types
+        // wider than 64 bits, nor dump(), which would also instantiate the
+        // serializer for objects and fail for key types it cannot write
+        using integer_limits = std::numeric_limits<typename BasicJsonType::number_integer_t>;
+        using unsigned_limits = std::numeric_limits<typename BasicJsonType::number_unsigned_t>;
+        const std::string number = j.is_number_unsigned()
+                                   ? integer_to_string(j.m_data.m_value.number_unsigned, std::integral_constant<bool, unsigned_limits::is_signed>())
+                                   : integer_to_string(j.m_data.m_value.number_integer, std::integral_constant<bool, integer_limits::is_signed>());
+        static_cast<void>(number); // unused when JSON_NOEXCEPTION is defined
+        static_cast<void>(format_name);
+        static_cast<void>(range);
+        JSON_THROW(out_of_range::create(exception_id::integer_too_large, concat("integer number ", number, " cannot be represented by ", format_name, " as it does not fit ", range), &j));
+    }
+
     void write_msgpack_array_prefix(const std::size_t N, const BasicJsonType& j)
     {
         const auto n = to_msgpack_length(N, j);
@@ -1580,6 +1651,8 @@ class binary_writer
             is neither an object nor an array
     @throw out_of_range.415 if @a j is binary with a subtype that does not fit
            into a byte, before anything is written
+    @throw out_of_range.407 if @a j is an integer that does not fit the 64 bits
+           of a BSON integer, before anything is written
     @throw type_error.316 if @a j is a string that is not valid UTF-8, before
            anything is written
     @throw type_error.321 if @a j is discarded
@@ -1598,9 +1671,17 @@ class binary_writer
                 return 8ul;
 
             case value_t::number_integer:
-                return calc_bson_integer_size(j.m_data.m_value.number_integer);
+                if (JSON_HEDLEY_UNLIKELY(!integer_fits<std::int64_t>(j.m_data.m_value.number_integer)))
+                {
+                    throw_integer_out_of_range(j, "BSON", "int64");
+                }
+                return calc_bson_integer_size(static_cast<std::int64_t>(j.m_data.m_value.number_integer));
 
             case value_t::number_unsigned:
+                if (JSON_HEDLEY_UNLIKELY(!integer_fits_max<std::uint64_t>(j.m_data.m_value.number_unsigned)))
+                {
+                    throw_integer_out_of_range(j, "BSON", "uint64");
+                }
                 return calc_bson_unsigned_size(conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
 
             case value_t::string:
@@ -1639,8 +1720,9 @@ class binary_writer
             case value_t::number_float:
                 return write_bson_double(name, j.m_data.m_value.number_float);
 
+            // calc_bson_value_size() checked that integers fit 64 bits
             case value_t::number_integer:
-                return write_bson_integer(name, j.m_data.m_value.number_integer);
+                return write_bson_integer(name, static_cast<std::int64_t>(j.m_data.m_value.number_integer));
 
             case value_t::number_unsigned:
                 return write_bson_unsigned(name, conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
@@ -1951,6 +2033,376 @@ class binary_writer
         }
     }
 
+    /*!
+    @brief size class of an unsigned integer from its bit length
+
+    1, 2, 3, or 4 if @a x fits 1, 2, 4, or 8 bytes. Looked up from the number
+    of leading zero bits instead of found by branches: the sizes of a stream
+    of numbers are hard to predict, and a mispredicted branch costs more than
+    writing the number. (A table, as comparisons are vectorized by clang into
+    slower code.)
+    */
+    static unsigned integer_size_class(const std::uint64_t x) noexcept
+    {
+        static constexpr std::array<std::uint8_t, 64> size_class_by_leading_zeros =
+        {
+            {
+                4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
+                3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
+                2, 2, 2, 2, 2, 2, 2, 2,
+                1, 1, 1, 1, 1, 1, 1, 1
+            }
+        };
+        // x | 1 keeps count_leading_zeros() away from 0
+        return size_class_by_leading_zeros[static_cast<std::size_t>(count_leading_zeros(x | 1u))];
+    }
+
+    /*!
+    @brief encode @a first_byte and the low-order bytes of @a value that
+           @a size_class calls for (none, or 1, 2, 4, or 8) in network order
+           to @a out
+    @return the number of characters of the encoding; up to
+            @ref integer_encoding_space characters at @a out are overwritten
+
+    The bytes are written as one byte and one 8-byte word whose bytes after
+    the value are scratch that the caller drops, so that writing them does not
+    depend on the size class. For class 0 the word is all scratch; it is
+    shifted by 60 bits rather than 64, which would be undefined.
+    */
+    std::size_t encode_first_byte_and_tail(CharType* out, const std::uint8_t first_byte, const unsigned size_class, const std::uint64_t value) const noexcept
+    {
+        out[0] = to_char_type(first_byte);
+        const unsigned unused_bits = 64u - ((8u << size_class) >> 1u);
+        const std::uint64_t tail = is_little_endian ? byte_swap(value) >> unused_bits : value << unused_bits;
+        std::memcpy(out + 1, &tail, sizeof(tail));
+        return ((std::size_t{1} << size_class) >> 1u) + 1;
+    }
+
+    /// characters encode_first_byte_and_tail() may overwrite: the first byte
+    /// and one 8-byte word
+    static constexpr std::size_t integer_encoding_space = 9;
+
+    /*!
+    @brief the 64 bits of a number_integer or number_unsigned value, and a mask
+           that is all ones if it is negative, else zero
+
+    Selected through masks rather than branches, as the type and the sign of
+    the values in an array of mixed signs alternate unpredictably (non-negative
+    integers are number_unsigned after parsing). Up to 64 bits, the sign is bit
+    63 of the result (a narrower integer is sign-extended); computing it with
+    masks keeps GCC from branching on the type. A wider integer has to be
+    compared.
+    */
+    static std::uint64_t integer_bits(const BasicJsonType& j, std::uint64_t& sign_mask) noexcept
+    {
+        const bool is_signed = j.m_data.m_type == value_t::number_integer;
+        const std::uint64_t raw = is_signed
+                                  ? static_cast<std::uint64_t>(j.m_data.m_value.number_integer)
+                                  : static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned);
+        sign_mask = sizeof(typename BasicJsonType::number_integer_t) <= sizeof(std::uint64_t)
+                    ? (std::uint64_t{0} - static_cast<std::uint64_t>(is_signed)) & (std::uint64_t{0} - (raw >> 63u))
+                    : std::uint64_t{0} - static_cast<std::uint64_t>(is_signed && j.m_data.m_value.number_integer < 0);
+        return raw;
+    }
+
+    /*!
+    @brief encode the head of a CBOR integer to @a out without branching on its
+           size
+    @return the number of characters of the head
+
+    Same output as @ref write_cbor_head. The lengths of strings and containers
+    are mostly predictable, so their heads keep the branches; the sizes of a
+    run of integers are not, so an integer's head is assembled without them.
+    */
+    std::size_t encode_cbor_integer_head(CharType* out, const std::uint8_t major_type, const std::uint64_t argument) const noexcept
+    {
+        // an argument of at most 23 is in size class 1 (one byte) and is moved
+        // to class 0, the first byte, by subtracting 1
+        const auto is_immediate = static_cast<unsigned>(argument <= 0x17u);
+        const unsigned size_class = integer_size_class(argument) - is_immediate;
+
+        // the first byte: the argument itself if it is immediate, else
+        // 0x17 + size_class, selected through a mask
+        const std::uint64_t immediate_mask = std::uint64_t{0} - is_immediate;
+        const auto first_byte = static_cast<std::uint8_t>(major_type + ((argument & immediate_mask) | ((0x17u + size_class) & ~immediate_mask)));
+        return encode_first_byte_and_tail(out, first_byte, size_class, argument);
+    }
+
+    /*!
+    @brief write an array value (head and elements) as CBOR
+
+    Integer elements, the most common scalar, are written without a call to
+    @ref write_cbor. The loop is kept out of line: inlined into write_cbor, its
+    registers would be saved and restored on every call of write_cbor, which
+    is one call per value nested in an object.
+    */
+    JSON_HEDLEY_NEVER_INLINE
+    void write_cbor_array(const BasicJsonType& j, const std::size_t depth)
+    {
+        write_cbor_head(0x80, j.m_data.m_value.array->size());
+        write_cbor_array_elements(j, depth, std::integral_constant<bool, integers_fit_64_bits()>());
+    }
+
+    void write_cbor_array_elements(const BasicJsonType& j, const std::size_t depth, std::true_type /*fits_64_bits*/)
+    {
+        write_array_elements(j, [this](CharType * out, const BasicJsonType & el)
+        {
+            return encode_cbor_integer(out, el);
+        }, [this, depth](const BasicJsonType & el)
+        {
+            write_cbor(el, depth + 1);
+        });
+    }
+
+    void write_cbor_array_elements(const BasicJsonType& j, const std::size_t depth, std::false_type /*fits_64_bits*/)
+    {
+        for (const auto& el : *j.m_data.m_value.array)
+        {
+            write_cbor(el, depth + 1);
+        }
+    }
+
+    /*!
+    @brief write the elements of the array @a j, runs of integers with
+           @a encode_integer and all other values with @a write_value
+
+    Runs of integers are encoded straight into space reserved from the sink
+    for a whole run: the write pointer stays a local, where it would otherwise
+    be reloaded from the sink for every value, as a store of a character may
+    alias it.
+    */
+    template<typename EncodeInteger, typename WriteValue>
+    void write_array_elements(const BasicJsonType& j, EncodeInteger encode_integer, WriteValue write_value)
+    {
+        constexpr std::size_t max_run = max_reserve_size / integer_encoding_space;
+        auto it = j.m_data.m_value.array->begin();
+        const auto end = j.m_data.m_value.array->end();
+        while (it != end)
+        {
+            if (it->is_number_integer())
+            {
+                CharType* const start = oa.reserve(max_run * integer_encoding_space);
+                CharType* out = start;
+                for (std::size_t n = 0; n < max_run && it != end && it->is_number_integer(); ++n, ++it)
+                {
+                    out += encode_integer(out, *it);
+                }
+                oa.commit(static_cast<std::size_t>(out - start));
+            }
+            else
+            {
+                write_value(*it);
+                ++it;
+            }
+        }
+    }
+
+    /*!
+    @brief write a number_integer or number_unsigned value as a CBOR integer
+
+    A negative integer n has major type 1 and the argument -1 - n, which is ~n.
+    Kept out of line, like the other per-value paths, so that write_cbor stays
+    small (runs of integers in an array are encoded inline instead).
+    */
+    JSON_HEDLEY_NEVER_INLINE
+    void write_cbor_integer(const BasicJsonType& j, std::true_type /*fits_64_bits*/)
+    {
+        oa.commit(encode_cbor_integer(oa.reserve(integer_encoding_space), j));
+    }
+
+    /// write a number_integer or number_unsigned value of a number type wider
+    /// than 64 bits as a CBOR integer, with range checks on the whole value
+    void write_cbor_integer(const BasicJsonType& j, std::false_type /*fits_64_bits*/)
+    {
+        if (j.m_data.m_type == value_t::number_unsigned)
+        {
+            if (JSON_HEDLEY_UNLIKELY(!integer_fits_max<std::uint64_t>(j.m_data.m_value.number_unsigned)))
+            {
+                throw_integer_out_of_range(j, "CBOR", "uint64");
+            }
+            write_cbor_head(0x00, conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+            return;
+        }
+
+        if (j.m_data.m_value.number_integer >= 0)
+        {
+            if (JSON_HEDLEY_UNLIKELY(!integer_fits_max<std::uint64_t>(j.m_data.m_value.number_integer)))
+            {
+                throw_integer_out_of_range(j, "CBOR", "[-2^64, 2^64-1]");
+            }
+            // CBOR does not differentiate between positive signed
+            // integers and unsigned integers
+            write_cbor_head(0x00, static_cast<std::uint64_t>(j.m_data.m_value.number_integer));
+        }
+        else
+        {
+            // a negative integer n is encoded as -1 - n
+            if (JSON_HEDLEY_UNLIKELY(!integer_fits_max<std::uint64_t>(-1 - j.m_data.m_value.number_integer)))
+            {
+                throw_integer_out_of_range(j, "CBOR", "[-2^64, 2^64-1]");
+            }
+            write_cbor_head(0x20, static_cast<std::uint64_t>(-1 - j.m_data.m_value.number_integer));
+        }
+    }
+
+    /// encode a number_integer or number_unsigned value to @a out (see
+    /// @ref write_cbor_integer) and return the number of characters
+    std::size_t encode_cbor_integer(CharType* out, const BasicJsonType& j) const noexcept
+    {
+        std::uint64_t sign_mask = 0;
+        const std::uint64_t raw = integer_bits(j, sign_mask);
+        // ~raw is -1 - n in the 64 bits that are written
+        return encode_cbor_integer_head(out, static_cast<std::uint8_t>(sign_mask & 0x20u), raw ^ sign_mask);
+    }
+
+    /*!
+    @brief whether the CBOR and MessagePack integers of BasicJsonType are
+           written by @ref encode_cbor_integer and @ref encode_msgpack_integer
+
+    These pick the format from the low 64 bits; a wider number type is written
+    with range checks on the whole value instead.
+    */
+    static constexpr bool integers_fit_64_bits() noexcept
+    {
+        return sizeof(typename BasicJsonType::number_integer_t) <= sizeof(std::uint64_t)
+               && sizeof(typename BasicJsonType::number_unsigned_t) <= sizeof(std::uint64_t);
+    }
+
+    /*!
+    @brief write a number_integer or number_unsigned value as a MessagePack
+           integer without branching on its sign or size
+
+    Same output as @ref write_msgpack: a non-negative integer as positive fixint
+    (< 128) or uint 8/16/32/64, a negative one as negative fixint (>= -32) or
+    int 8/16/32/64. Kept out of line (see @ref write_cbor_integer).
+    */
+    JSON_HEDLEY_NEVER_INLINE
+    void write_msgpack_integer(const BasicJsonType& j, std::true_type /*fits_64_bits*/)
+    {
+        oa.commit(encode_msgpack_integer(oa.reserve(integer_encoding_space), j));
+    }
+
+    /// encode a number_integer or number_unsigned value to @a out (see
+    /// @ref write_msgpack_integer) and return the number of characters
+    std::size_t encode_msgpack_integer(CharType* out, const BasicJsonType& j) const noexcept
+    {
+        std::uint64_t sign_mask = 0;
+        const std::uint64_t raw = integer_bits(j, sign_mask);
+
+        // A negative n fits int 8/16/32 exactly if 2 * (-1 - n) + 1 fits
+        // uint 8/16/32, so x gives the size class of either sign.
+        const std::uint64_t is_negative = sign_mask & 1u;
+        const std::uint64_t x = ((raw ^ sign_mask) << is_negative) | is_negative;
+
+        // A fixint is at most 127, or at least -32 (x at most 63). It is in
+        // size class 1 and is moved to class 0, the first byte, by subtracting 1.
+        const auto is_fixint = static_cast<unsigned>(x <= (127u ^ (sign_mask & 64u)));
+        const unsigned size_class = integer_size_class(x) - is_fixint;
+
+        // the first byte: the low byte of the integer for a fixint, else the
+        // marker 0xCC..0xCF (uint) or 0xD0..0xD3 (int), selected through a mask
+        const std::uint64_t fixint_mask = std::uint64_t{0} - is_fixint;
+        const std::uint64_t marker = 0xCBu + size_class + (sign_mask & 4u);
+        const auto first_byte = static_cast<std::uint8_t>((raw & fixint_mask) | (marker & ~fixint_mask));
+        return encode_first_byte_and_tail(out, first_byte, size_class, raw);
+    }
+
+    /// write a number_integer or number_unsigned value of a number type wider
+    /// than 64 bits as a MessagePack integer, with range checks on the whole value
+    void write_msgpack_integer(const BasicJsonType& j, std::false_type /*fits_64_bits*/)
+    {
+        if (j.m_data.m_type == value_t::number_unsigned)
+        {
+            if (JSON_HEDLEY_UNLIKELY(!integer_fits_max<std::uint64_t>(j.m_data.m_value.number_unsigned)))
+            {
+                throw_integer_out_of_range(j, "MessagePack", "uint64");
+            }
+            write_msgpack_unsigned(static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+            return;
+        }
+
+        if (j.m_data.m_value.number_integer >= 0)
+        {
+            if (JSON_HEDLEY_UNLIKELY(!integer_fits_max<std::uint64_t>(j.m_data.m_value.number_integer)))
+            {
+                throw_integer_out_of_range(j, "MessagePack", "[-2^63, 2^64-1]");
+            }
+            // MessagePack does not differentiate between positive
+            // signed integers and unsigned integers.
+            write_msgpack_unsigned(static_cast<std::uint64_t>(j.m_data.m_value.number_integer));
+        }
+        else
+        {
+            if (JSON_HEDLEY_UNLIKELY(!integer_fits_min<std::int64_t>(j.m_data.m_value.number_integer)))
+            {
+                throw_integer_out_of_range(j, "MessagePack", "[-2^63, 2^64-1]");
+            }
+            if (j.m_data.m_value.number_integer >= -32)
+            {
+                // negative fixnum
+                write_number(static_cast<std::int8_t>(j.m_data.m_value.number_integer));
+            }
+            else if (j.m_data.m_value.number_integer >= (std::numeric_limits<std::int8_t>::min)() &&
+                     j.m_data.m_value.number_integer <= (std::numeric_limits<std::int8_t>::max)())
+            {
+                // int 8
+                oa.write_character(to_char_type(0xD0));
+                write_number(static_cast<std::int8_t>(j.m_data.m_value.number_integer));
+            }
+            else if (j.m_data.m_value.number_integer >= (std::numeric_limits<std::int16_t>::min)() &&
+                     j.m_data.m_value.number_integer <= (std::numeric_limits<std::int16_t>::max)())
+            {
+                // int 16
+                oa.write_character(to_char_type(0xD1));
+                write_number(static_cast<std::int16_t>(j.m_data.m_value.number_integer));
+            }
+            else if (j.m_data.m_value.number_integer >= (std::numeric_limits<std::int32_t>::min)() &&
+                     j.m_data.m_value.number_integer <= (std::numeric_limits<std::int32_t>::max)())
+            {
+                // int 32
+                oa.write_character(to_char_type(0xD2));
+                write_number(static_cast<std::int32_t>(j.m_data.m_value.number_integer));
+            }
+            else
+            {
+                // int 64
+                oa.write_character(to_char_type(0xD3));
+                write_number(static_cast<std::int64_t>(j.m_data.m_value.number_integer));
+            }
+        }
+    }
+
+    /*!
+    @brief write an array value (head and elements) as MessagePack
+
+    See @ref write_cbor_array.
+    */
+    JSON_HEDLEY_NEVER_INLINE
+    void write_msgpack_array(const BasicJsonType& j, const std::size_t depth)
+    {
+        write_msgpack_array_prefix(j.m_data.m_value.array->size(), j);
+        write_msgpack_array_elements(j, depth, std::integral_constant<bool, integers_fit_64_bits()>());
+    }
+
+    void write_msgpack_array_elements(const BasicJsonType& j, const std::size_t depth, std::true_type /*fits_64_bits*/)
+    {
+        write_array_elements(j, [this](CharType * out, const BasicJsonType & el)
+        {
+            return encode_msgpack_integer(out, el);
+        }, [this, depth](const BasicJsonType & el)
+        {
+            write_msgpack(el, depth + 1);
+        });
+    }
+
+    void write_msgpack_array_elements(const BasicJsonType& j, const std::size_t depth, std::false_type /*fits_64_bits*/)
+    {
+        for (const auto& el : *j.m_data.m_value.array)
+        {
+            write_msgpack(el, depth + 1);
+        }
+    }
+
     /*!
     @brief write an object key as a CBOR text string
 
@@ -2147,7 +2599,7 @@ class binary_writer
         {
             return 'L';
         }
-        if (use_bjdata && std::is_unsigned<NumberType>::value)
+        if (use_bjdata && std::is_unsigned<NumberType>::value && integer_fits_max<std::uint64_t>(n))
         {
             return 'M';
         }
@@ -2272,14 +2724,16 @@ class binary_writer
     @brief checks whether a JSON number fits into @a TargetType
     @param[in] el a JSON number of either the signed or unsigned integer kind
     @return whether @a el's value can be represented by @a TargetType without
-            wrapping, regardless of which of the two kinds it is stored as
+            wrapping, regardless of which of the two kinds it is stored as;
+            false for a value that does not even fit the 64-bit type it is
+            read as (possible for number types wider than 64 bits)
     */
     template<typename TargetType>
     static bool bjdata_ndarray_value_in_range(const BasicJsonType& el)
     {
         return el.is_number_unsigned()
-               ? value_in_range_of<TargetType>(el.template get<std::uint64_t>())
-               : value_in_range_of<TargetType>(el.template get<std::int64_t>());
+               ? integer_fits_max<std::uint64_t>(el.m_data.m_value.number_unsigned) && value_in_range_of<TargetType>(el.template get<std::uint64_t>())
+               : integer_fits<std::int64_t>(el.m_data.m_value.number_integer) && value_in_range_of<TargetType>(el.template get<std::int64_t>());
     }
 
     /*!
@@ -2511,10 +2965,11 @@ class binary_writer
         for (const auto& el : dims)
         {
             // a dimension is read as an unsigned value below, so anything that
-            // is not a non-negative integer is rejected: a non-integer entry
-            // would pun unrelated bytes as the dimension, and a negative one
-            // would wrap into a nonsensical length
-            if (!el.is_number_integer() || (!el.is_number_unsigned() && el.template get<std::int64_t>() < 0))
+            // is not a non-negative integer in the range of std::uint64_t is
+            // rejected: a non-integer entry would pun unrelated bytes as the
+            // dimension, and a negative or wider one would wrap into a
+            // nonsensical length
+            if (!el.is_number_integer() || !bjdata_ndarray_value_in_range<std::uint64_t>(el))
             {
                 return true;
             }
@@ -2646,9 +3101,9 @@ class binary_writer
 
             case value_t::number_unsigned:
             {
-                if (j.m_data.m_value.number_unsigned > static_cast<typename BasicJsonType::number_unsigned_t>((std::numeric_limits<std::int64_t>::max)()))
+                if (JSON_HEDLEY_UNLIKELY(!integer_fits_max<std::int64_t>(j.m_data.m_value.number_unsigned)))
                 {
-                    JSON_THROW(out_of_range::create(exception_id::integer_too_large, concat("integer number ", std::to_string(j.m_data.m_value.number_unsigned), " cannot be represented by BON8 as it does not fit int64"), &j));
+                    throw_integer_out_of_range(j, "BON8", "int64");
                 }
                 write_bon8_integer(static_cast<std::int64_t>(j.m_data.m_value.number_unsigned));
                 string_open = false;
@@ -2657,6 +3112,10 @@ class binary_writer
 
             case value_t::number_integer:
             {
+                if (JSON_HEDLEY_UNLIKELY(!integer_fits<std::int64_t>(j.m_data.m_value.number_integer)))
+                {
+                    throw_integer_out_of_range(j, "BON8", "int64");
+                }
                 write_bon8_integer(static_cast<std::int64_t>(j.m_data.m_value.number_integer));
                 string_open = false;
                 break;

include/nlohmann/detail/output/output_adapters.hpp

@@ -8,7 +8,10 @@
 
 #pragma once
 
+#include <algorithm> // max
+#include <array> // array
 #include <cstddef> // size_t
+#include <cstring> // memcpy
 #include <memory> // shared_ptr, make_shared
 #include <string> // basic_string
 #include <type_traits> // conditional, integral_constant, is_same
@@ -48,32 +51,170 @@ template<typename CharType> struct output_adapter_protocol
 template<typename CharType>
 using output_adapter_t = std::shared_ptr<output_adapter_protocol<CharType>>;
 
+/// the largest number of characters a writer may reserve() from a sink at once;
+/// output_vector_sink also appends writes of at least this size with insert()
+constexpr std::size_t max_reserve_size = 256;
+
 /// @brief non-virtual output sink writing into a std::vector
 ///
 /// This sink is not part of the virtual output_adapter_protocol hierarchy: it is
 /// passed to binary_writer by value as a template parameter, so
 /// write_character()/write_characters() are ordinary (inlinable) calls with no
 /// vtable lookup and no shared_ptr. It is used for the common
-/// `to_cbor`/`to_msgpack`/... into a std::vector. output_vector_adapter below
-/// wraps this same sink to provide the virtual interface.
+/// `to_cbor`/`to_msgpack`/... into a std::vector.
+///
+/// Most writes are a few bytes - a type marker, or a number of 1, 2, 4, or 8
+/// bytes - and appending those with push_back() or insert() cost more than
+/// encoding them: insert() is an out-of-line call into a routine that can insert
+/// anywhere, and resize() is one as well with libstdc++. The sink therefore keeps
+/// its own write position: it grows the vector geometrically ahead of the
+/// position and copies each write to it, so a write is a capacity check and a
+/// short copy. The destructor shrinks the vector to the bytes actually written,
+/// also when a write throws; shrinking a vector never allocates. The sink owns
+/// that trim, so it can be moved but not copied, and the vector must not be used
+/// while the sink is alive.
 template<typename CharType, typename AllocatorType = std::allocator<CharType>>
 class output_vector_sink
 {
   public:
     explicit output_vector_sink(std::vector<CharType, AllocatorType>& vec) noexcept
+        : v(&vec), position(vec.size())
+    {}
+
+    output_vector_sink(const output_vector_sink&) = delete;
+    output_vector_sink& operator=(const output_vector_sink&) = delete;
+    output_vector_sink& operator=(output_vector_sink&&) = delete;
+
+    output_vector_sink(output_vector_sink&& other) noexcept
+        : v(other.v), position(other.position)
+    {
+        other.v = nullptr;
+    }
+
+    ~output_vector_sink()
+    {
+        if (v != nullptr)
+        {
+            v->resize(position);
+        }
+    }
+
+    // always inlined: GCC otherwise keeps these out of line in a large
+    // translation unit, and a call per byte or number costs more than the write
+    JSON_HEDLEY_ALWAYS_INLINE
+    void write_character(CharType c)
+    {
+        if (JSON_HEDLEY_UNLIKELY(position == v->size()))
+        {
+            grow(1);
+        }
+        (*v)[position++] = c;
+    }
+
+    // no JSON_HEDLEY_NON_NULL here: binary_writer legitimately passes a null
+    // pointer with length 0 for empty strings/binary values; memcpy must not
+    // see it.
+    JSON_HEDLEY_ALWAYS_INLINE
+    void write_characters(const CharType* s, std::size_t length)
+    {
+        if (JSON_HEDLEY_UNLIKELY(v->size() - position < length))
+        {
+            if (length >= max_reserve_size)
+            {
+                append(s, length);
+                return;
+            }
+            grow(length);
+        }
+        if (length != 0)
+        {
+            std::memcpy(v->data() + position, s, length * sizeof(CharType));
+            position += length;
+        }
+    }
+
+    /// @brief space for writing up to @a length characters directly
+    ///
+    /// Returns a pointer to at least @a length (at most @ref max_reserve_size)
+    /// writable characters at the write position; @ref commit then keeps the
+    /// first ones of them. A writer can store whole words there and keep only
+    /// the bytes it needs, and keep its write pointer in a register across many
+    /// values instead of updating the sink after each.
+    JSON_HEDLEY_ALWAYS_INLINE
+    CharType* reserve(const std::size_t length)
+    {
+        JSON_ASSERT(length <= max_reserve_size);
+        if (JSON_HEDLEY_UNLIKELY(v->size() - position < length))
+        {
+            grow(length);
+        }
+        return v->data() + position;
+    }
+
+    /// keep the first @a length characters written to the space from reserve()
+    JSON_HEDLEY_ALWAYS_INLINE
+    void commit(const std::size_t length) noexcept
+    {
+        position += length;
+    }
+
+  private:
+    /// make room for at least @a length more characters after the position;
+    /// kept out of line so that the write functions stay small enough for
+    /// GCC to inline them into the writer
+    JSON_HEDLEY_NEVER_INLINE
+    void grow(const std::size_t length)
+    {
+        // use the capacity the vector already has, else at least double it;
+        // never less than 64 characters, so that a small value does not
+        // reallocate for every few bytes
+        const std::size_t needed = position + length;
+        v->resize((std::max)(std::size_t{64}, needed <= v->capacity()
+                             ? v->capacity()
+                             : (std::max)(needed, v->capacity() * 2)));
+    }
+
+    /// append @a length (at least max_reserve_size) characters that do not fit
+    /// the space ahead of the position: insert() copies them once into the
+    /// grown vector, where grow() would first zero the new space and then copy
+    /// over it - noticeable for a large string or binary value
+    JSON_HEDLEY_NEVER_INLINE
+    void append(const CharType* s, const std::size_t length)
+    {
+        v->resize(position);
+        if (v->capacity() - position < length)
+        {
+            // grow geometrically, with room for the writes that follow, so
+            // that the next grow() need not reallocate right away
+            v->reserve((std::max)(v->capacity() * 2, position + length + max_reserve_size));
+        }
+        v->insert(v->end(), s, s + length);
+        position = v->size();
+    }
+
+    std::vector<CharType, AllocatorType>* v;
+    /// number of characters of *v written so far
+    std::size_t position;
+};
+
+/// output adapter for byte vectors
+template<typename CharType, typename AllocatorType = std::allocator<CharType>>
+class output_vector_adapter : public output_adapter_protocol<CharType>
+{
+  public:
+    explicit output_vector_adapter(std::vector<CharType, AllocatorType>& vec) noexcept
         : v(vec)
     {}
 
-    void write_character(CharType c)
+    void write_character(CharType c) override
     {
         v.push_back(c);
     }
 
     // no JSON_HEDLEY_NON_NULL here: binary_writer legitimately passes a null
-    // pointer with length 0 for empty strings/binary values. Appending an empty
-    // range is a no-op; the type-erased path tolerates this via the (unattributed)
-    // virtual base, and the concrete sink must do the same.
-    void write_characters(const CharType* s, std::size_t length)
+    // pointer with length 0 for empty strings/binary values; appending an empty
+    // range is a no-op
+    void write_characters(const CharType* s, std::size_t length) override
     {
         v.insert(v.end(), s, s + length);
     }
@@ -82,33 +223,6 @@ class output_vector_sink
     std::vector<CharType, AllocatorType>& v;
 };
 
-/// output adapter for byte vectors
-///
-/// The appending itself lives in output_vector_sink; this class only adds the
-/// virtual output_adapter_protocol interface on top of it, so both the
-/// type-erased and the templated path share one implementation.
-template<typename CharType, typename AllocatorType = std::allocator<CharType>>
-class output_vector_adapter : public output_adapter_protocol<CharType>
-{
-  public:
-    explicit output_vector_adapter(std::vector<CharType, AllocatorType>& vec) noexcept
-        : sink(vec)
-    {}
-
-    void write_character(CharType c) override
-    {
-        sink.write_character(c);
-    }
-
-    void write_characters(const CharType* s, std::size_t length) override
-    {
-        sink.write_characters(s, length);
-    }
-
-  private:
-    output_vector_sink<CharType, AllocatorType> sink;
-};
-
 #ifndef JSON_NO_IO
 /// output adapter for output streams
 template<typename CharType>
@@ -188,8 +302,24 @@ class output_adapter_sink
         oa->write_characters(s, length);
     }
 
+    /// space for writing up to @a length characters (see
+    /// output_vector_sink::reserve)
+    CharType* reserve(const std::size_t length) noexcept
+    {
+        JSON_ASSERT(length <= max_reserve_size);
+        static_cast<void>(length);
+        return reserved.data();
+    }
+
+    /// write the first @a length characters written to the space from reserve()
+    void commit(const std::size_t length)
+    {
+        oa->write_characters(reserved.data(), length);
+    }
+
   private:
     output_adapter_t<CharType> oa;
+    std::array<CharType, max_reserve_size> reserved{};
 };
 
 /// @brief whether std::basic_string<CharType> has a non-deprecated std::char_traits

include/nlohmann/detail/output/serializer.hpp

@@ -1525,7 +1525,7 @@ class serializer
         // without leading zeros
         if (abs_value < 10000000000000000u)
         {
-            const std::uint64_t value = abs_value;
+            const auto value = static_cast<std::uint64_t>(abs_value); // fits: abs_value < 10^16
             const std::uint64_t upper = value / 100000000u;
             const std::uint64_t first = dtoa_impl::eight_digit_bytes(upper != 0 ? upper : value);
             const auto leading = static_cast<unsigned>(count_leading_zeros(first) / 8); // (first is not 0)

include/nlohmann/detail/value_t.hpp

@@ -14,7 +14,6 @@
 #include <cstdint> // uint8_t
 #include <limits> // numeric_limits
 #include <string> // string
-#include <type_traits> // is_signed
 
 #include <nlohmann/detail/macro_scope.hpp>
 #if JSON_HAS_THREE_WAY_COMPARISON
@@ -208,13 +207,16 @@ FloatType compare_integer_with_float(const IntegerType i, const FloatType f) noe
 
     // values of IntegerType lie in [-bound, bound) when signed and in
     // [0, bound) when unsigned; digits excludes the sign bit, so bound is a
-    // power of two that the float represents exactly
-    const FloatType bound = std::ldexp(static_cast<FloatType>(1), std::numeric_limits<IntegerType>::digits);
+    // power of two that the float represents exactly; the signedness comes
+    // from numeric_limits as well, because std::is_signed is false for class
+    // types such as 128-bit or multiprecision integers
+    using limits = std::numeric_limits<IntegerType>;
+    const FloatType bound = std::ldexp(static_cast<FloatType>(1), limits::digits);
     if (f >= bound)
     {
         return ordered(-1);
     }
-    if (std::is_signed<IntegerType>::value ? (f < -bound) : (f < static_cast<FloatType>(0)))
+    if (limits::is_signed ? (f < -bound) : (f < static_cast<FloatType>(0)))
     {
         return ordered(1);
     }

include/nlohmann/detail/view/document_data.hpp

@@ -10,7 +10,7 @@
 
 #include <array> // array
 #include <cstddef> // size_t
-#include <cstdint> // uint32_t
+#include <cstdint> // uint8_t, uint32_t
 #include <cstring> // memcmp, memcpy
 #include <limits> // numeric_limits
 #include <map> // map
@@ -42,7 +42,9 @@ struct document_data
     std::size_t inline_capacity = 0;
     std::size_t max_depth = 0; ///< deepest nesting of arrays and objects (the root container is depth 1)
     std::vector<char> arena; ///< decoded strings that contained escapes (no terminator: they are handed out with their length)
+    std::size_t arena_size = 0; ///< bytes of decoded strings at base[1] (the arena, or those of a loaded image)
     std::string owned; ///< owned copy of the input, if any (a std::string, so that its NUL serves as the parser's sentinel)
+    std::vector<std::uint8_t> owned_image; ///< a loaded image the document owns (the text and the decoded strings point into it)
 
     // hash indexes of the keys of large objects (see object_index.hpp)
     static constexpr std::uint32_t index_min_members = 128;
@@ -119,6 +121,7 @@ struct document_data
     document_data() noexcept
         : arena() // NOLINT(readability-redundant-member-init)
         , owned() // NOLINT(readability-redundant-member-init)
+        , owned_image() // NOLINT(readability-redundant-member-init)
         , indexes() // NOLINT(readability-redundant-member-init)
         , index_slots() // NOLINT(readability-redundant-member-init)
         , large_objects() // NOLINT(readability-redundant-member-init)

include/nlohmann/detail/view/image.hpp

@@ -0,0 +1,744 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <algorithm> // min, sort
+#include <array> // array
+#include <cstddef> // offsetof, size_t
+#include <cstdint> // int64_t, uint8_t, uint16_t, uint32_t, uint64_t
+#include <cstring> // memcmp, memcpy, memset
+#include <limits> // numeric_limits
+#include <string> // string
+#include <vector> // vector
+
+#include <nlohmann/json.hpp>
+#include <nlohmann/detail/view/document_data.hpp>
+#include <nlohmann/detail/view/errors.hpp>
+#include <nlohmann/detail/view/macro_scope.hpp>
+#include <nlohmann/detail/view/node.hpp>
+#include <nlohmann/detail/view/number.hpp>
+#include <nlohmann/detail/view/object_index.hpp>
+#include <nlohmann/detail/view/scan.hpp>
+
+// Images: a document stored so that loading it needs no parsing.
+//
+// Layout (little-endian): a 64-byte header, the nodes, the text (the source,
+// followed by the number tokens written by edits), a NUL, the decoded strings
+// (followed by the strings written by edits), a NUL. The idea is that of
+// zero-copy formats such as FlatBuffers (https://github.com/google/flatbuffers)
+// and YaFF (https://github.com/yandex/yaff); no code is taken from them.
+// check_image follows the idea of FlatBuffers' Verifier (bounds and
+// structure) and also checks what the parser guarantees about strings and
+// numbers, so that reading and serializing a checked image is safe and yields
+// valid JSON.
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+namespace view
+{
+
+/// how load() checks an image
+enum class image_check
+{
+    /// everything the parser guarantees: structure and bounds, strings (valid
+    /// UTF-8; source strings without quotes, backslashes, and control
+    /// characters), and numbers (well-formed, matching the stored values)
+    full,
+    /// structure and bounds only: reading and serializing are safe, but a
+    /// crafted image can yield invalid UTF-8, strings that serialize to
+    /// invalid JSON, or numbers that differ from their text
+    bounds,
+    /// none: for images from a trusted source only (a damaged image is
+    /// undefined behavior)
+    none,
+};
+
+struct image_header
+{
+    std::array<char, 4> magic;              ///< "NJVI"
+    std::uint32_t version;                  ///< 1
+    std::uint64_t node_count;
+    std::uint64_t text_size;
+    std::uint64_t arena_size;
+    std::array<std::uint64_t, 4> reserved;  ///< zero (for later versions)
+};
+static_assert(sizeof(image_header) == 64, "the image header must be 64 bytes");
+
+constexpr std::uint32_t image_version = 1;
+
+/// the largest node count and text or string size of an image (as for parsed
+/// documents, offsets and counts must fit 32 bits)
+constexpr std::uint64_t image_limit = 0xFFFFFFF0u;
+
+/// Copy the current structure of an edited document into nodes in document
+/// order, as the parser would have written them. Text written by edits is
+/// appended to text_tail (number tokens) and arena_tail (strings); floats that
+/// are not finite become null, as dump() writes them.
+inline void compact_nodes(const document_data& d, std::size_t arena_size, std::vector<node>& out, std::string& text_tail, std::string& arena_tail)
+{
+    struct frame
+    {
+        const node* cur;
+        const node* end;
+        std::size_t index; ///< the container's node in out
+        std::uint32_t count;
+        bool object;
+    };
+    std::vector<frame> stack;
+    const auto string_node = [&](const node & s)
+    {
+        node r = s;
+        r.extra = 0;
+        r.flags = static_cast<std::uint8_t>(s.flags & node_flags::storage);
+        if (r.flags == node_flags::edited)
+        {
+            r.off = static_cast<std::uint32_t>(arena_size + arena_tail.size());
+            arena_tail.append(d.str(s), s.len);
+            r.flags = node_flags::escaped;
+        }
+        return r;
+    };
+    const auto copy_node = [&](const node * v)
+    {
+        node r = *v;
+        switch (static_cast<value_t>(v->kind))
+        {
+            case value_t::object:
+            case value_t::array:
+                r.flags = 0;
+                r.extra = 0;
+                r.off = (v->flags & (node_flags::moved | node_flags::is_new)) != 0 ? 0 : v->off;
+                r.len = 0;  // counted below
+                r.next = 0; // set when the container is complete
+                stack.push_back(frame{d.first_child_edited(v), d.child_end_edited(v), out.size(), 0, v->kind == static_cast<std::uint8_t>(value_t::object)});
+                break;
+            case value_t::string:
+                r = string_node(*v);
+                break;
+            case value_t::number_integer:
+            case value_t::number_unsigned:
+                if ((v->flags & node_flags::storage) == node_flags::edited)
+                {
+                    r.off = static_cast<std::uint32_t>(d.size + text_tail.size());
+                    text_tail.append(d.str(*v), number_length(*v));
+                }
+                r.flags = 0;
+                break;
+            case value_t::number_float:
+                if ((v->flags & node_flags::storage) == node_flags::edited)
+                {
+                    const char* const t = d.str(*v);
+                    if (t[0] == 'n' || t[0] == 'i' || (v->len > 1 && t[1] == 'i'))
+                    {
+                        r = node{}; // nan and infinity: null, as dump() writes them
+                        r.kind = static_cast<std::uint8_t>(value_t::null);
+                        break;
+                    }
+                    r.off = static_cast<std::uint32_t>(d.size + text_tail.size());
+                    text_tail.append(t, v->len);
+                    r.extra = 0xFFFFu; // the digit layout is not recorded
+                }
+                r.flags = 0;
+                break;
+            case value_t::boolean:
+                r.flags = static_cast<std::uint8_t>(v->flags & node_flags::is_true);
+                break;
+            case value_t::null:
+            case value_t::binary:
+            case value_t::discarded:
+            default:
+                r.flags = 0;
+                break;
+        }
+        out.push_back(r);
+    };
+    copy_node(d.nodes);
+    while (!stack.empty())
+    {
+        const frame top = stack.back();
+        if (top.cur == top.end)
+        {
+            node& c = out[top.index];
+            c.len = top.count;
+            c.next = static_cast<std::uint32_t>(out.size() - top.index);
+            stack.pop_back();
+            continue;
+        }
+        if (top.object)
+        {
+            out.push_back(string_node(*top.cur));
+        }
+        const node* const value = top.cur + (top.object ? 1 : 0);
+        stack.back().cur = document_data::after(value);
+        ++stack.back().count;
+        copy_node(document_data::deref(value)); // may grow the stack
+    }
+}
+
+/// the document as an image
+inline std::vector<std::uint8_t> save_image(const document_data& d)
+{
+#if !NLOHMANN_VIEW_LITTLE_ENDIAN
+    throw_type_error(320, "json_document images need a little-endian target"); // LCOV_EXCL_LINE
+#endif
+    const std::size_t arena_size = d.arena_size;
+    std::vector<node> compacted;
+    std::string text_tail;
+    std::string arena_tail;
+    if (d.edits)
+    {
+        compact_nodes(d, arena_size, compacted, text_tail, arena_tail);
+    }
+    const node* const nodes = d.edits ? compacted.data() : d.nodes;
+    const std::size_t count = d.edits ? compacted.size() : d.node_count;
+    const std::size_t text_size = d.size + text_tail.size();
+    const std::size_t total_arena = arena_size + arena_tail.size();
+    if (NLOHMANN_VIEW_UNLIKELY(text_size >= image_limit || total_arena >= image_limit || count >= image_limit))
+    {
+        // LCOV_EXCL_START (4 GiB)
+        throw_out_of_range(416, "images of 4 GiB or more are not supported by json_document");
+        // LCOV_EXCL_STOP
+    }
+    image_header h{};
+    h.magic = {{'N', 'J', 'V', 'I'}};
+    h.version = image_version;
+    h.node_count = count;
+    h.text_size = text_size;
+    h.arena_size = total_arena;
+    std::vector<std::uint8_t> image(sizeof(h) + (count * sizeof(node)) + text_size + 1 + total_arena + 1);
+    std::uint8_t* o = image.data();
+    std::memcpy(o, &h, sizeof(h));
+    o += sizeof(h);
+    std::memcpy(o, nodes, count * sizeof(node));
+    // the hash indexes are rebuilt by load(), whose check rejects any other
+    // extra of an object, and the same document saves to the same bytes
+    for (std::size_t i = 0; i < count; ++i)
+    {
+        if (nodes[i].kind == static_cast<std::uint8_t>(value_t::object) && nodes[i].extra != 0)
+        {
+            std::memset(o + (i * sizeof(node)) + offsetof(node, extra), 0, sizeof(nodes[i].extra));
+        }
+    }
+    o += count * sizeof(node);
+    const auto append = [&o](const char* s, std::size_t n)
+    {
+        if (n != 0)
+        {
+            std::memcpy(o, s, n);
+            o += n;
+        }
+    };
+    append(d.src, d.size);
+    append(text_tail.data(), text_tail.size());
+    *o++ = 0;
+    append(d.base[1], arena_size);
+    append(arena_tail.data(), arena_tail.size());
+    *o = 0;
+    return image;
+}
+
+/// the parts of a number token that the checks need
+struct number_token
+{
+    const unsigned char* int_start;
+    std::size_t int_digits;
+    std::size_t frac_digits;
+    std::int64_t exponent;
+    bool negative;
+    bool is_float; ///< a fraction or an exponent
+};
+
+/// whether [s, s + len) is a JSON number (the grammar the parser accepts)
+inline bool scan_number_token(const unsigned char* s, std::size_t len, number_token& t)
+{
+    const unsigned char* const e = s + len;
+    const unsigned char* p = s;
+    t.negative = p != e && *p == '-';
+    p += t.negative ? 1 : 0;
+    if (p == e)
+    {
+        return false;
+    }
+    t.int_start = p;
+    if (*p == '0')
+    {
+        ++p;
+    }
+    else if (*p >= '1' && *p <= '9')
+    {
+        do
+        {
+            ++p;
+        }
+        while (p != e && is_digit(*p));
+    }
+    else
+    {
+        return false;
+    }
+    t.int_digits = static_cast<std::size_t>(p - t.int_start);
+    t.frac_digits = 0;
+    t.is_float = false;
+    if (p != e && *p == '.')
+    {
+        const unsigned char* const f0 = ++p;
+        while (p != e && is_digit(*p))
+        {
+            ++p;
+        }
+        if (p == f0)
+        {
+            return false;
+        }
+        t.frac_digits = static_cast<std::size_t>(p - f0);
+        t.is_float = true;
+    }
+    t.exponent = 0;
+    if (p != e && (*p | 0x20u) == 'e')
+    {
+        ++p;
+        const bool exp_negative = p != e && *p == '-';
+        p += (p != e && (*p == '+' || *p == '-')) ? 1 : 0;
+        if (p == e || !is_digit(*p))
+        {
+            return false;
+        }
+        do
+        {
+            t.exponent = t.exponent < 100000 ? (t.exponent * 10) + (*p - '0') : t.exponent;
+            ++p;
+        }
+        while (p != e && is_digit(*p));
+        t.exponent = exp_negative ? -t.exponent : t.exponent;
+        t.is_float = true;
+    }
+    return p == e;
+}
+
+/// the digit layout the parser records for a float token (compaction
+/// writes "many" instead; the caller accepts both)
+inline std::uint16_t float_layout(const number_token& t)
+{
+    return static_cast<std::uint16_t>((std::min)(t.int_digits, std::size_t{255}) | ((std::min)(t.frac_digits, std::size_t{255}) << 8u));
+}
+
+/// whether a float token (well-formed, per scan_number_token) is finite as
+/// double; parse() rejects floats that overflow, and as there, only a number
+/// whose magnitude could reach 1e308 needs the conversion
+inline bool float_token_finite(const unsigned char* s, std::size_t len, const number_token& t)
+{
+    if (static_cast<std::int64_t>(t.int_digits) + t.exponent > 300)
+    {
+        node n{};
+        n.kind = static_cast<std::uint8_t>(value_t::number_float);
+        n.len = static_cast<std::uint32_t>(len);
+        const auto v = float_value<double>(reinterpret_cast<const char*>(s), n); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+        return v <= (std::numeric_limits<double>::max)() && v >= -(std::numeric_limits<double>::max)();
+    }
+    return true;
+}
+
+/// whether an integer node matches its token the way the parser records it
+/// (after the bounds check; the token has at most 256 characters)
+inline bool check_integer(const node& n, const unsigned char* text)
+{
+    const std::size_t len = number_length(n);
+    const unsigned char* const s = text + n.off;
+    number_token t{};
+    if (!scan_number_token(s, len, t))
+    {
+        return false;
+    }
+    // the token's value is the stored one; number_integer nodes of edits can
+    // be non-negative (as basic_json keeps the type of a value)
+    const bool integer = n.kind == static_cast<std::uint8_t>(value_t::number_integer);
+    if (t.is_float || t.int_digits > 20 || (t.negative && !integer))
+    {
+        return false;
+    }
+    // (at most 19 digits cannot overflow; 20 digits are compared with 2^64 - 1)
+    if (t.int_digits == 20 && std::memcmp(t.int_start, "18446744073709551615", 20) > 0)
+    {
+        return false;
+    }
+    std::uint64_t m = 0;
+    for (const unsigned char* d = t.int_start; d != t.int_start + t.int_digits; ++d)
+    {
+        m = (m * 10) + static_cast<std::uint64_t>(*d - '0');
+    }
+    if (integer && m > (t.negative ? std::uint64_t{1} << 63u : (std::uint64_t{1} << 63u) - 1))
+    {
+        return false;
+    }
+    return integer_bits(n) == (t.negative ? 0 - m : m);
+}
+
+/// a byte range of the text or of the decoded strings
+struct byte_range
+{
+    std::uint32_t off;
+    std::uint32_t len;
+};
+
+/// a float token and the digit layout its node records
+struct float_range
+{
+    std::uint32_t off;
+    std::uint32_t len;
+    std::uint16_t extra;
+};
+
+/// Run check once for each distinct range of ranges (which are within bounds
+/// and not empty). Ranges that are not identical must not overlap: save()
+/// writes every string and token to a place of its own, and nodes that share
+/// a value (copies within a document) share the whole range. This bounds the
+/// work by the size of the text, however many nodes point to the same bytes.
+template<typename Check>
+bool check_distinct_ranges(std::vector<byte_range>& ranges, Check check)
+{
+    std::sort(ranges.begin(), ranges.end(), [](const byte_range & a, const byte_range & b)
+    {
+        return a.off != b.off ? a.off < b.off : a.len < b.len;
+    });
+    std::size_t end = 0;
+    for (std::size_t i = 0; i < ranges.size(); ++i)
+    {
+        const byte_range r = ranges[i];
+        if (i != 0 && r.off == ranges[i - 1].off && r.len == ranges[i - 1].len)
+        {
+            continue;
+        }
+        if (r.off < end || !check(r))
+        {
+            return false;
+        }
+        end = static_cast<std::size_t>(r.off) + r.len;
+    }
+    return true;
+}
+
+/// Check the float nodes: each token once (nodes of the same token must
+/// record layouts that match it), tokens must not overlap.
+inline bool check_float_ranges(std::vector<float_range>& ranges, const unsigned char* text)
+{
+    std::sort(ranges.begin(), ranges.end(), [](const float_range & a, const float_range & b)
+    {
+        if (a.off != b.off)
+        {
+            return a.off < b.off;
+        }
+        if (a.len != b.len)
+        {
+            return a.len < b.len;
+        }
+        return a.extra < b.extra;
+    });
+    std::size_t end = 0;
+    std::size_t i = 0;
+    while (i < ranges.size())
+    {
+        const float_range r = ranges[i];
+        if (r.off < end)
+        {
+            return false;
+        }
+        number_token t{};
+        const unsigned char* const s = text + r.off;
+        if (!scan_number_token(s, r.len, t) || !float_token_finite(s, r.len, t))
+        {
+            return false;
+        }
+        // the digit layout the parser records (or "many", as compaction writes it)
+        const std::uint16_t layout = float_layout(t);
+        for (; i < ranges.size() && ranges[i].off == r.off && ranges[i].len == r.len; ++i)
+        {
+            if (ranges[i].extra != layout && ranges[i].extra != 0xFFFFu)
+            {
+                return false;
+            }
+        }
+        end = static_cast<std::size_t>(r.off) + r.len;
+    }
+    return true;
+}
+
+/// the content checks of check_image: source strings as the parser leaves
+/// them (no quotes, backslashes, or control characters), decoded strings
+/// (valid UTF-8), and float tokens
+inline bool check_contents(const unsigned char* text, const unsigned char* arena,
+                           std::vector<byte_range>& source_strings, std::vector<byte_range>& decoded_strings,
+                           std::vector<float_range>& floats)
+{
+    return check_distinct_ranges(source_strings, [text](const byte_range & r)
+    {
+        const unsigned char* const b = text + r.off;
+        return scan_string_run(b, b + r.len) == b + r.len;
+    })
+    && check_distinct_ranges(decoded_strings, [arena](const byte_range & r)
+    {
+        return valid_utf8_prefix(arena + r.off, r.len) == r.len;
+    })
+    && check_float_ranges(floats, text);
+}
+
+/// Check the nodes of a loaded image against its text and decoded strings:
+/// kinds, flags, and `extra`; extents and element counts of arrays and
+/// objects; keys; bounds; string contents (source strings as the parser
+/// leaves them: no quotes, backslashes, or control characters; all strings
+/// valid UTF-8); and number tokens.
+///
+/// The structure and the bounds are checked node by node. The contents of
+/// strings and of float tokens are checked afterwards, once for each distinct
+/// range (see check_distinct_ranges), so that the full check is linear in the
+/// size of the image plus the sorting of the ranges, and not in the number of
+/// nodes times the size of the text.
+inline bool check_image(const node* nodes, std::size_t count, const unsigned char* text, std::size_t text_size,
+                        const unsigned char* arena, std::size_t arena_size, bool full)
+{
+    struct frame
+    {
+        std::size_t end;
+        std::uint32_t len;
+        std::uint32_t seen;
+        bool object;
+        bool expect_key;
+    };
+    std::vector<frame> stack;
+    std::vector<byte_range> source_strings;
+    std::vector<byte_range> decoded_strings;
+    std::vector<float_range> floats;
+    const auto check_string = [&](const node & n) -> bool
+    {
+        if ((n.flags & ~node_flags::escaped) != 0 || n.extra != 0)
+        {
+            return false;
+        }
+        const bool decoded = (n.flags & node_flags::escaped) != 0;
+        const std::size_t limit = decoded ? arena_size : text_size;
+        if (n.off > limit || n.len > limit - n.off)
+        {
+            return false;
+        }
+        if (full && n.len != 0)
+        {
+            (decoded ? decoded_strings : source_strings).push_back(byte_range{n.off, n.len});
+        }
+        return true;
+    };
+    // bounds of a number token; the recorded digit layout must lie within it
+    const auto number_in_bounds = [&](const node & n) -> bool
+    {
+        const std::size_t len = number_length(n);
+        if (len == 0 || n.off > text_size || len > text_size - n.off)
+        {
+            return false;
+        }
+        if (n.kind != static_cast<std::uint8_t>(value_t::number_float))
+        {
+            return (n.extra >> 8u) == 0;
+        }
+        // float_value() reads the sign, the integer digits, and the point and
+        // fraction digits the layout records (a layout of more than 19 digits
+        // means the general conversion, which stays within the token)
+        const std::size_t int_digits = n.extra & 0xFFu;
+        const std::size_t frac_digits = n.extra >> 8u;
+        const std::size_t need = (text[n.off] == '-' ? 1u : 0u) + int_digits + (frac_digits != 0 ? frac_digits + 1 : 0);
+        return int_digits + frac_digits > 19 || need <= len;
+    };
+    std::size_t i = 0;
+    for (;;)
+    {
+        // close finished arrays and objects
+        while (!stack.empty() && i == stack.back().end)
+        {
+            const frame f = stack.back();
+            if (f.seen != f.len || (f.object && !f.expect_key))
+            {
+                return false;
+            }
+            stack.pop_back();
+            if (!stack.empty())
+            {
+                ++stack.back().seen;
+                stack.back().expect_key = true;
+            }
+        }
+        if (i == count)
+        {
+            return stack.empty() && (!full || check_contents(text, arena, source_strings, decoded_strings, floats));
+        }
+        if (i != 0 && stack.empty())
+        {
+            return false; // nodes after the root
+        }
+        const node& n = nodes[i];
+        if (!stack.empty() && stack.back().object && stack.back().expect_key)
+        {
+            if (n.kind != static_cast<std::uint8_t>(value_t::string) || !check_string(n))
+            {
+                return false;
+            }
+            stack.back().expect_key = false;
+            ++i;
+            continue;
+        }
+        bool complete = true;
+        switch (static_cast<value_t>(n.kind))
+        {
+            case value_t::null:
+                // (the offset of a literal is read to size the output of dump())
+                if (n.flags != 0 || n.extra != 0 || n.off > text_size)
+                {
+                    return false;
+                }
+                break;
+            case value_t::boolean:
+                if ((n.flags & ~node_flags::is_true) != 0 || n.extra != 0 || n.off > text_size)
+                {
+                    return false;
+                }
+                break;
+            case value_t::string:
+                if (!check_string(n))
+                {
+                    return false;
+                }
+                break;
+            case value_t::number_integer:
+            case value_t::number_unsigned:
+            case value_t::number_float:
+                if (n.flags != 0 || !number_in_bounds(n))
+                {
+                    return false;
+                }
+                if (full)
+                {
+                    // (the contents of float tokens are checked later; the token of an
+                    // integer has at most 256 characters)
+                    if (n.kind == static_cast<std::uint8_t>(value_t::number_float))
+                    {
+                        floats.push_back(float_range{n.off, n.len, n.extra});
+                    }
+                    else if (!check_integer(n, text))
+                    {
+                        return false;
+                    }
+                }
+                break;
+            case value_t::array:
+            case value_t::object:
+            {
+                const std::size_t limit = stack.empty() ? count : stack.back().end;
+                if (n.flags != 0 || n.extra != 0 || n.next == 0 || n.next > limit - i || n.off > text_size)
+                {
+                    return false;
+                }
+                stack.push_back(frame{i + n.next, n.len, 0, n.kind == static_cast<std::uint8_t>(value_t::object), true});
+                complete = false;
+                break;
+            }
+            case value_t::binary:
+            case value_t::discarded:
+            default:
+                return false;
+        }
+        ++i;
+        if (complete && !stack.empty())
+        {
+            ++stack.back().seen;
+            stack.back().expect_key = true;
+        }
+    }
+}
+
+[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_invalid_image(const char* what)
+{
+    throw_parse_error(116, concat("invalid json_document image: ", what));
+}
+
+/// Read an image into d. The text and the decoded strings stay in the image;
+/// the nodes are copied (so that they are aligned, and edits can change them).
+inline void load_image(document_data& d, const std::uint8_t* image, std::size_t size, image_check check)
+{
+#if !NLOHMANN_VIEW_LITTLE_ENDIAN
+    throw_type_error(320, "json_document images need a little-endian target"); // LCOV_EXCL_LINE
+#endif
+    if (image == nullptr || size < sizeof(image_header))
+    {
+        throw_invalid_image("too short");
+    }
+    image_header h{};
+    std::memcpy(&h, image, sizeof(h));
+    // (the reserved fields are for later versions)
+    if (std::memcmp(h.magic.data(), "NJVI", 4) != 0 || h.version != image_version
+            || (h.reserved[0] | h.reserved[1] | h.reserved[2] | h.reserved[3]) != 0)
+    {
+        throw_invalid_image("unknown format");
+    }
+    const std::size_t room = size - sizeof(h);
+    if (h.node_count == 0 || h.node_count > room / sizeof(node) || h.node_count >= image_limit || h.text_size >= image_limit || h.arena_size >= image_limit)
+    {
+        throw_invalid_image("sizes out of range");
+    }
+    const auto count = static_cast<std::size_t>(h.node_count);
+    const auto text_size = static_cast<std::size_t>(h.text_size);
+    const auto arena_size = static_cast<std::size_t>(h.arena_size);
+    const std::size_t text_at = sizeof(h) + (count * sizeof(node));
+    // the text, a NUL, the decoded strings, a NUL, and nothing after them
+    if (size - text_at < 2 || text_size > size - text_at - 2 || arena_size != size - text_at - text_size - 2
+            || image[text_at + text_size] != 0 || image[size - 1] != 0)
+    {
+        throw_invalid_image("sizes out of range");
+    }
+
+    d.discarded = true;
+    d.edits.reset();
+    d.base[2] = nullptr;
+    d.owned.clear();
+    if (d.owned_image.empty() || image != d.owned_image.data())
+    {
+        d.owned_image.clear();
+    }
+    d.arena.clear();
+    d.indexes.clear();
+    d.index_slots.clear();
+    d.large_objects.clear();
+    d.node_count = 0;
+    d.reserve(count);
+    std::memcpy(d.nodes, image + sizeof(h), count * sizeof(node));
+    d.node_count = count;
+    const std::uint8_t* const text = image + text_at;
+    const std::uint8_t* const arena = text + text_size + 1;
+    d.src = reinterpret_cast<const char*>(text); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+    d.size = text_size;
+    d.base[0] = d.src;
+    d.base[1] = reinterpret_cast<const char*>(arena); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+    d.arena_size = arena_size;
+    if (check != image_check::none && !check_image(d.nodes, count, text, text_size, arena, arena_size, check == image_check::full))
+    {
+        throw_invalid_image("the check failed");
+    }
+    // the hash indexes of large objects, as after parsing
+    for (std::size_t i = 0; i < count; ++i)
+    {
+        const node& n = d.nodes[i];
+        if (n.kind == static_cast<std::uint8_t>(value_t::object) && n.len >= document_data::index_min_members)
+        {
+            d.large_objects.push_back(static_cast<std::uint32_t>(i));
+        }
+    }
+    build_object_indexes(d);
+    std::vector<std::uint32_t>().swap(d.large_objects); // (only needed while building)
+    d.discarded = false;
+}
+
+}  // namespace view
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/detail/view/number.hpp

@@ -26,43 +26,79 @@ namespace detail
 namespace view
 {
 
+/*!
+@brief locate the decimal point and the end of the mantissa of a float token
+
+Also checks that the token is a JSON number. Tokens of the parser and of edits
+always are; an image loaded with image_check::bounds can hold any bytes, which
+must not reach the conversion (it expects a well-formed token).
+*/
+inline bool float_token_layout(const char* first, const char* last, std::size_t& dot, std::size_t& mantissa_end) noexcept
+{
+    const auto digit = [last](const char* q)
+    {
+        return q != last && is_digit(static_cast<unsigned char>(*q));
+    };
+    const char* p = first;
+    p += (p != last && *p == '-') ? 1 : 0;
+    if (!digit(p) || (*p == '0' && digit(p + 1)))
+    {
+        return false;
+    }
+    do
+    {
+        ++p;
+    }
+    while (digit(p));
+    dot = std::string::npos;
+    if (p != last && *p == '.')
+    {
+        dot = static_cast<std::size_t>(p - first);
+        if (!digit(++p))
+        {
+            return false;
+        }
+        do
+        {
+            ++p;
+        }
+        while (digit(p));
+    }
+    mantissa_end = static_cast<std::size_t>(p - first);
+    if (p != last && (*p == 'e' || *p == 'E'))
+    {
+        ++p;
+        p += (p != last && (*p == '+' || *p == '-')) ? 1 : 0;
+        if (!digit(p))
+        {
+            return false;
+        }
+        do
+        {
+            ++p;
+        }
+        while (digit(p));
+    }
+    return p == last;
+}
+
 /*!
 @brief the value of the float token of a node, as parse() converts it
 
 Uses the lexer's conversion (detail::convert_float), so that the values are
 bit-identical to parse(): float and double are converted without allocation
-and independent of the locale. The digit layout recorded while parsing locates
-the decimal point and the exponent without scanning the token.
+and independent of the locale. A token that is not a JSON number (only in a
+damaged image loaded with image_check::bounds) yields 0.
 */
 template<typename FloatType>
 NLOHMANN_VIEW_NOINLINE FloatType float_value(const char* first, const node& n)
 {
     const char* const last = first + n.len;
-    const std::size_t neg = first[0] == '-' ? 1 : 0;
-    const std::size_t int_digits = n.extra & 0xFFu;
-    const std::size_t frac_digits = n.extra >> 8u;
-    std::size_t dot = std::string::npos;
-    std::size_t mantissa_end = n.len;
-    if (int_digits != 255 && frac_digits != 255)
+    std::size_t dot = 0;
+    std::size_t mantissa_end = 0;
+    if (NLOHMANN_VIEW_UNLIKELY(!float_token_layout(first, last, dot, mantissa_end)))
     {
-        dot = frac_digits != 0 ? neg + int_digits : std::string::npos;
-        mantissa_end = neg + int_digits + (frac_digits != 0 ? 1 + frac_digits : 0);
-    }
-    else
-    {
-        // more digits than the layout records: locate them
-        for (std::size_t i = 0; i < n.len; ++i)
-        {
-            if (first[i] == '.')
-            {
-                dot = i;
-            }
-            else if (first[i] == 'e' || first[i] == 'E')
-            {
-                mantissa_end = i;
-                break;
-            }
-        }
+        return FloatType{};
     }
     return convert_float<FloatType>(first, last, dot, mantissa_end);
 }
@@ -93,16 +129,20 @@ NLOHMANN_VIEW_ALWAYS_INLINE float_significand layout_decimal(const unsigned char
     }
     if (p != e)
     {
-        // [eE][+-]digits; huge exponents saturate (the parser rejected overflow)
+        // [eE][+-]digits; huge exponents saturate (the parser rejected
+        // overflow). The token is not read beyond e, and the digits are taken
+        // as unsigned, so that a token that is not well-formed (a damaged
+        // image loaded with image_check::bounds) yields a wrong value, but no
+        // overflow.
         ++p;
-        const bool exp_negative = *p == '-';
-        p += (*p == '-' || *p == '+') ? 1 : 0;
+        const bool exp_negative = p != e && *p == '-';
+        p += (p != e && (*p == '-' || *p == '+')) ? 1 : 0;
         std::int64_t exp_value = 0;
         for (; p != e; ++p)
         {
             if (exp_value < 0x10000000)
             {
-                exp_value = (exp_value * 10) + (*p - '0');
+                exp_value = (exp_value * 10) + static_cast<unsigned char>(*p - '0');
             }
         }
         q += exp_negative ? -exp_value : exp_value;

include/nlohmann/detail/view/scan.hpp

@@ -175,32 +175,7 @@ NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* skip_digits(const unsigned char
 /// powers of ten up to 10^19 as integers
 inline std::uint64_t int_pow10(unsigned k) noexcept
 {
-    static const std::array<std::uint64_t, 20> table =
-    {
-        {
-            1u,
-            10u,
-            100u,
-            1000u,
-            10000u,
-            100000u,
-            1000000u,
-            10000000u,
-            100000000u,
-            1000000000u,
-            10000000000u,
-            100000000000u,
-            1000000000000u,
-            10000000000000u,
-            100000000000000u,
-            1000000000000000u,
-            10000000000000000u,
-            100000000000000000u,
-            1000000000000000000u,
-            10000000000000000000u
-        }
-    };
-    return table[k];
+    return ::nlohmann::detail::dtoa_impl::powers_of_ten()[k];
 }
 
 /// value of 0 < k < 8 digits at p in one step if [p, p + 8) lies below

include/nlohmann/detail/view/serializer.hpp

@@ -31,7 +31,11 @@ namespace view
 {
 
 /// append-only output buffer: writes through a raw pointer into a string that
-/// is resized ahead, and trimmed by finish()
+/// is resized ahead, and trimmed by finish(). The estimate is reserved, and the
+/// string grows in steps of 64 KiB within it: resize() fills the new bytes
+/// with zeros (before C++23, a string cannot grow without), and a small step
+/// is filled while the writer is about to use it, in the cache, instead of
+/// filling the whole estimate in memory first.
 template<typename StringType>
 class output_buffer
 {
@@ -83,17 +87,49 @@ class output_buffer
         m_pos += n;
     }
 
+    /// the write position and the end of the writable space, for a writer
+    /// that keeps the position in a local variable (set_cursor() hands it back)
+    char* cursor() const noexcept
+    {
+        return m_pos;
+    }
+
+    char* limit() const noexcept
+    {
+        return m_end;
+    }
+
+    void set_cursor(char* p) noexcept
+    {
+        m_pos = p;
+    }
+
   private:
+    /// the size of a growth step (a function: std::min() takes a reference,
+    /// which a static constexpr member does not have before C++17)
+    static constexpr std::size_t step() noexcept
+    {
+        return 0x10000;
+    }
+
     static StringType& sized(StringType& out, std::size_t estimate)
     {
-        out.resize((std::max)(estimate, static_cast<std::size_t>(64)));
+        out.reserve(estimate);
+        out.resize((std::min)((std::max)(estimate, static_cast<std::size_t>(64)), step()));
         return out;
     }
 
     NLOHMANN_VIEW_NOINLINE void grow(std::size_t n)
     {
         const auto used = static_cast<std::size_t>(m_pos - m_out.data());
-        m_out.resize((std::max)(m_out.size() * 2, used + n + 256));
+        // (a step does not go beyond the reserved estimate, so that a good
+        // estimate is never copied to a larger allocation)
+        const std::size_t size = (std::max)((std::min)(m_out.size() + step(), m_out.capacity()), used + n + 256);
+        if (size > m_out.capacity())
+        {
+            m_out.reserve((std::max)(m_out.capacity() * 2, size));
+        }
+        m_out.resize(size);
         m_pos = &m_out[0] + used;
         m_end = &m_out[0] + m_out.size();
     }
@@ -103,6 +139,117 @@ class output_buffer
     char* m_end;
 };
 
+/// The length of the run at s that dump() writes unchanged without
+/// ensure_ascii: all bytes but quotes, backslashes, and control characters.
+/// Unlike detail::string_bulk_run(), non-ASCII bytes are not validated: the
+/// strings of a document are valid UTF-8 (a damaged image loaded with
+/// image_check::bounds can have others, which are then written unchanged).
+inline std::size_t plain_output_run(const unsigned char* s, std::size_t n) noexcept
+{
+    constexpr std::uint64_t ones = 0x0101010101010101ull;
+    constexpr std::uint64_t high = 0x8080808080808080ull;
+    std::size_t i = 0;
+    for (; i + 8 <= n; i += 8)
+    {
+        const std::uint64_t v = read_eight_bytes(s + i);
+        const std::uint64_t q = v ^ 0x2222222222222222ull; // '"'
+        const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\'
+        const std::uint64_t stop = (((q - ones) & ~q) | ((b - ones) & ~b) | ((v - 0x2020202020202020ull) & ~v)) & high;
+        if (stop != 0)
+        {
+            // the lowest flagged byte is the first stop: borrows only flag bytes above a true one
+            return i + (static_cast<std::size_t>(count_trailing_zeros(stop)) / 8);
+        }
+    }
+    for (; i < n; ++i)
+    {
+        if (s[i] == '"' || s[i] == '\\' || s[i] < 0x20)
+        {
+            return i;
+        }
+    }
+    return n;
+}
+
+/// A stack that starts in a buffer of the caller (a local array) and moves to
+/// the heap (a vector of the caller) only when that is full, so that dumps of
+/// shallow documents need no allocation. The top is a pointer, as in
+/// std::vector. grow() is static and out of line: it gets the vector and the
+/// used range and returns the new storage, so the address of the stack never
+/// reaches a call. Otherwise the compiler would have to keep the three
+/// pointers in memory and load and store them on every push_back() and
+/// pop_back(), not just when growing.
+template<typename T>
+class small_stack
+{
+  public:
+    small_stack(T* buffer, std::size_t capacity, std::vector<T>& heap) noexcept
+        : m_begin(buffer), m_top(buffer), m_end(buffer + capacity), m_heap(heap)
+    {}
+    small_stack(const small_stack&) = delete;
+    small_stack(small_stack&&) = delete;
+    small_stack& operator=(const small_stack&) = delete;
+    small_stack& operator=(small_stack&&) = delete;
+    ~small_stack() = default;
+
+    NLOHMANN_VIEW_ALWAYS_INLINE void push_back(const T& x)
+    {
+        if (NLOHMANN_VIEW_UNLIKELY(m_top == m_end))
+        {
+            const std::size_t used = size();
+            const std::size_t capacity = 2 * static_cast<std::size_t>(m_end - m_begin);
+            m_begin = grow(m_heap, m_begin, used, capacity);
+            m_top = m_begin + used;
+            m_end = m_begin + capacity;
+        }
+        *m_top++ = x;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE T& back() noexcept
+    {
+        return m_top[-1];
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE void pop_back() noexcept
+    {
+        --m_top;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE bool empty() const noexcept
+    {
+        return m_top == m_begin;
+    }
+
+    NLOHMANN_VIEW_ALWAYS_INLINE std::size_t size() const noexcept
+    {
+        return static_cast<std::size_t>(m_top - m_begin);
+    }
+
+  private:
+    /// the used entries moved to heap storage of the given capacity
+    NLOHMANN_VIEW_NOINLINE static T* grow(std::vector<T>& heap, const T* begin, std::size_t used, std::size_t capacity)
+    {
+        std::vector<T> bigger(capacity);
+        std::copy(begin, begin + used, bigger.begin());
+        heap.swap(bigger);
+        return heap.data();
+    }
+
+    T* m_begin;
+    T* m_top;
+    T* m_end;
+    std::vector<T>& m_heap;
+};
+
+/// a string literal at w, without its terminating NUL (after room())
+/// @return a pointer past it
+template<std::size_t N>
+NLOHMANN_VIEW_ALWAYS_INLINE char* put_literal(char* w, const char (&s)[N]) noexcept // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
+{
+    std::memcpy(w, s, N - 1);
+    return w + N - 1;
+}
+
 /// how the view's dump() writes a value
 struct dump_style
 {
@@ -129,6 +276,8 @@ class view_serializer
     using nav = navigation<Editable>;
     using string_t = typename BasicJsonType::string_t;
     using number_float_t = typename BasicJsonType::number_float_t;
+    /// doubles that are IEEE 754 binary64 are written from their bits (Zmij)
+    using fast_double = std::integral_constant < bool, std::is_same<number_float_t, double>::value && ::nlohmann::detail::dtoa_impl::is_binary64<double>::value >;
 
   public:
     view_serializer(const document_data& d, string_t& out, std::size_t estimate, const dump_style& style)
@@ -137,6 +286,18 @@ class view_serializer
 
     void dump(const node* root)
     {
+        if (!m_style.pretty && !m_style.ensure_ascii)
+        {
+            if (m_style.source_numbers)
+            {
+                dump_compact<true>(root);
+            }
+            else
+            {
+                dump_compact<false>(root);
+            }
+            return;
+        }
         struct frame
         {
             const node* pos; ///< next element, or key of the next member
@@ -144,7 +305,9 @@ class view_serializer
             bool object;
             bool first;  ///< nothing written yet
         };
-        std::vector<frame> stack;
+        std::array<frame, 32> buffer; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+        std::vector<frame> heap;
+        small_stack<frame> stack(buffer.data(), buffer.size(), heap);
         const node* n = root;
         for (;;)
         {
@@ -215,6 +378,293 @@ class view_serializer
     }
 
   private:
+    /// a string that is not a plain string of the source (decoded, or written
+    /// by an edit), without ensure_ascii: runs without characters to escape
+    /// are copied
+    NLOHMANN_VIEW_NOINLINE void write_decoded(const node& n)
+    {
+        const auto* const s = reinterpret_cast<const unsigned char*>(m_doc.str(n)); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+        m_out.put('"');
+        for (std::size_t i = 0; i < n.len;)
+        {
+            const std::size_t run = plain_output_run(s + i, n.len - i);
+            if (run != 0)
+            {
+                m_out.put(reinterpret_cast<const char*>(s + i), run); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+                i += run;
+                continue;
+            }
+            write_codepoint<false>(s[i], s + i, 1); // a quote, a backslash, or a control character
+            ++i;
+        }
+        m_out.put('"');
+    }
+
+    /// the copies of dump_compact() that are not fixed-size moves (long
+    /// strings, or near the end of the source); out of line, so that the
+    /// compiler does not merge the fixed-size moves into this call
+    NLOHMANN_VIEW_NOINLINE static void copy_long(char* to, const char* from, std::size_t n) noexcept
+    {
+        std::memcpy(to, from, n);
+    }
+
+    /// a compile-time option as a runtime condition: testing the template
+    /// argument directly makes a condition like `Editable && c` constant when
+    /// the option is off, which MSVC reports as C4127
+    static NLOHMANN_VIEW_ALWAYS_INLINE bool enabled(bool option) noexcept
+    {
+        return option;
+    }
+
+    /*!
+    @brief the compact output without ensure_ascii (the default dump())
+
+    The same walk as dump(), with the write position in a local variable
+    (stores through char pointers would otherwise force a reload of the
+    buffer's members after each one), and with strings and number tokens of
+    the source copied by fixed-size moves of 32 bytes where the source has
+    that many bytes left, instead of a library call per token. The buffer
+    keeps 64 bytes of slack for the overshoot.
+    */
+    template<bool SourceNumbers>
+    void dump_compact(const node* root)
+    {
+        struct frame
+        {
+            const node* pos; ///< (editable documents) next element, or key of the next member
+            const node* end;
+            bool object;
+        };
+        std::array<frame, 32> buffer; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init): written before read
+        std::vector<frame> heap;
+        small_stack<frame> stack(buffer.data(), buffer.size(), heap);
+        const char* const src = m_doc.src;
+        const char* const src_end = src + m_doc.size;
+        char* w = m_out.cursor();
+        char* lim = m_out.limit();
+        // room for n bytes and the slack
+        const auto room = [&](std::size_t n)
+        {
+            if (NLOHMANN_VIEW_UNLIKELY(static_cast<std::size_t>(lim - w) < n + 64))
+            {
+                m_out.set_cursor(w);
+                m_out.reserve(n + 64);
+                w = m_out.cursor();
+                lim = m_out.limit();
+            }
+        };
+        // copy n bytes of the source (after room(n))
+        const auto copy = [&](const char* from, std::size_t n)
+        {
+            if (n <= 32 && static_cast<std::size_t>(src_end - from) >= 32)
+            {
+                std::memcpy(w, from, 32);
+            }
+            else if (n <= 256 && static_cast<std::size_t>(src_end - from) >= n + 32)
+            {
+                for (std::size_t i = 0; i < n; i += 32)
+                {
+                    std::memcpy(w + i, from + i, 32);
+                }
+            }
+            else
+            {
+                copy_long(w, from, n);
+            }
+            w += n;
+        };
+        // a string that is not a plain string of the source (out of line, so
+        // that the cursor stays in a register here)
+        const auto escaped = [&](const node & n)
+        {
+            m_out.set_cursor(w);
+            write_decoded(n);
+            w = m_out.cursor();
+            lim = m_out.limit();
+        };
+
+        // Read-only documents: the elements of a container follow it in the
+        // node array, so the walk goes through the array in order, and a
+        // frame only needs the end of its container. Editable documents: the
+        // elements of a moved container live elsewhere, so a frame keeps the
+        // position of the next element (see navigation).
+        // The innermost open container is kept in registers (cur; end ==
+        // nullptr: none), the stack holds the ones around it.
+        frame cur{nullptr, nullptr, false};
+        const node* n = root;
+        for (;;)
+        {
+            // write the value at n (read-only documents: and advance n)
+            bool opened = false;
+            // false positive: n comes from nav::value(), which never returns null for a valid index
+            // @infer-ignore NULLPTR_DEREFERENCE
+            switch (static_cast<value_t>(n->kind))
+            {
+                case value_t::string:
+                    if ((n->flags & node_flags::storage) == 0)
+                    {
+                        room(n->len + 2);
+                        *w++ = '"';
+                        copy(src + n->off, n->len);
+                        *w++ = '"';
+                    }
+                    else
+                    {
+                        escaped(*n);
+                    }
+                    break;
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                {
+                    const std::uint32_t len = number_length(*n);
+                    room(len);
+                    if (enabled(Editable) && (n->flags & node_flags::storage) != 0)
+                    {
+                        copy_long(w, m_doc.str(*n), len); // a canonical token written by an edit
+                        w += len;
+                        break;
+                    }
+                    const char* const token = src + n->off;
+                    if (!enabled(SourceNumbers) && NLOHMANN_VIEW_UNLIKELY(len == 2 && token[0] == '-' && token[1] == '0'))
+                    {
+                        *w++ = '0'; // parse() reads -0 as the integer 0
+                    }
+                    else
+                    {
+                        copy(token, len);
+                    }
+                    break;
+                }
+                case value_t::number_float:
+                    if (enabled(SourceNumbers) && (n->flags & node_flags::storage) != node_flags::edited)
+                    {
+                        room(n->len);
+                        copy(src + n->off, n->len);
+                    }
+                    else if (enabled(fast_double::value))
+                    {
+                        room(64);
+                        w = write_double_at(w, *n);
+                    }
+                    else
+                    {
+                        m_out.set_cursor(w);
+                        write_float_node(*n);
+                        w = m_out.cursor();
+                        lim = m_out.limit();
+                    }
+                    break;
+                case value_t::boolean:
+                    room(8);
+                    if ((n->flags & node_flags::is_true) != 0)
+                    {
+                        w = put_literal(w, "true");
+                    }
+                    else
+                    {
+                        w = put_literal(w, "false");
+                    }
+                    break;
+                case value_t::object:
+                case value_t::array:
+                {
+                    const bool object = n->kind == static_cast<std::uint8_t>(value_t::object);
+                    room(8);
+                    if (n->len == 0)
+                    {
+                        w = object ? put_literal(w, "{}") : put_literal(w, "[]");
+                    }
+                    else
+                    {
+                        *w++ = object ? '{' : '[';
+                        stack.push_back(cur);
+                        if (Editable)
+                        {
+                            cur = frame{nav::first(m_doc, n), nav::end(m_doc, n), object};
+                        }
+                        else
+                        {
+                            cur = frame{nullptr, n + n->next, object};
+                        }
+                        opened = true;
+                    }
+                    break;
+                }
+                case value_t::null:
+                    room(8);
+                    w = put_literal(w, "null");
+                    break;
+                case value_t::binary:    // LCOV_EXCL_LINE (not in a document)
+                case value_t::discarded: // LCOV_EXCL_LINE
+                default:                 // LCOV_EXCL_LINE
+                    break;               // LCOV_EXCL_LINE
+            }
+            if (!Editable)
+            {
+                ++n; // the next node: the first element of an opened container, or the node after a scalar
+            }
+
+            // go to the next value: close finished containers, then separate
+            // (a container just opened has an element)
+            if (!opened)
+            {
+                for (;;)
+                {
+                    if (cur.end == nullptr)
+                    {
+                        m_out.set_cursor(w);
+                        m_out.finish();
+                        return;
+                    }
+                    if ((Editable ? cur.pos : n) != cur.end)
+                    {
+                        break;
+                    }
+                    room(1);
+                    *w++ = cur.object ? '}' : ']';
+                    cur = stack.back();
+                    stack.pop_back();
+                }
+                room(1);
+                *w++ = ',';
+            }
+            const node* const at = Editable ? cur.pos : n;
+            if (cur.object)
+            {
+                const node& key = *at;
+                if ((key.flags & node_flags::storage) == 0)
+                {
+                    room(key.len + 3);
+                    *w++ = '"';
+                    copy(src + key.off, key.len);
+                    w[0] = '"';
+                    w[1] = ':';
+                    w += 2;
+                }
+                else
+                {
+                    escaped(key);
+                    room(1);
+                    *w++ = ':';
+                }
+                if (Editable)
+                {
+                    n = nav::value(at + 1);
+                    cur.pos = document_data::after(at + 1);
+                }
+                else
+                {
+                    ++n;
+                }
+            }
+            else if (Editable)
+            {
+                n = nav::value(at);
+                cur.pos = document_data::after(at);
+            }
+        }
+    }
+
     void newline(std::size_t level)
     {
         if (m_style.pretty)
@@ -266,7 +716,7 @@ class view_serializer
                 }
                 else
                 {
-                    write_float(float_value<number_float_t>(m_doc, n));
+                    write_float_node(n);
                 }
                 break;
             case value_t::object:    // LCOV_EXCL_LINE (containers are written by dump())
@@ -278,6 +728,88 @@ class view_serializer
         }
     }
 
+    /// a float node as dump() writes it
+    void write_float_node(const node& n)
+    {
+        write_float_node(n, fast_double {});
+    }
+
+    void write_float_node(const node& n, std::false_type /*other*/)
+    {
+        write_float(float_value<number_float_t>(m_doc, n));
+    }
+
+    void write_float_node(const node& n, std::true_type /*double*/)
+    {
+        m_out.reserve(64);
+        m_out.set_cursor(write_double_at(m_out.cursor(), n));
+    }
+
+    /*!
+    @brief (doubles) the float at n as dump() writes it, at w (64 bytes of room)
+
+    A token of at most 15 significant digits is written from its digits,
+    without a conversion: two decimals of at most 15 digits are farther
+    apart than the rounding interval of a (normal) double (the argument
+    behind DBL_DIG), so the token's digits are the shortest ones of its
+    double, which the library's conversion writes (Zmij). Other tokens are
+    converted from the digits already read.
+    */
+    char* write_double_at(char* w, const node& n)
+    {
+        const unsigned int_digits = n.extra & 0xFFu;
+        const unsigned frac_digits = n.extra >> 8u;
+        if ((n.flags & node_flags::storage) != node_flags::edited && int_digits + frac_digits <= 19)
+        {
+            const auto* const first = reinterpret_cast<const unsigned char*>(m_doc.src + n.off); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+            const float_significand d = layout_decimal(first, first + n.len, int_digits, frac_digits, reinterpret_cast<const unsigned char*>(m_doc.src + m_doc.size)); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+            // (the exponent keeps the value far from subnormals and overflow)
+            if (d.w != 0 && d.w < 1000000000000000u && d.exponent >= -290 && d.exponent <= 290)
+            {
+                *w = '-';
+                w += d.negative ? 1 : 0;
+                // (without leading zeros, all digits of the token count; the
+                // check also keeps an image that was only checked for bounds,
+                // whose token may not be made of digits, from the counted
+                // overload)
+                const auto& powers = ::nlohmann::detail::dtoa_impl::powers_of_ten();
+                const unsigned count = int_digits + frac_digits;
+                return count - 1u < 15u && d.w >= powers[count - 1u] && d.w < powers[count]
+                       ? ::nlohmann::detail::dtoa_impl::write_short_decimal(w, d.w, static_cast<int>(count), static_cast<int>(d.exponent))
+                       : ::nlohmann::detail::dtoa_impl::write_short_decimal(w, d.w, static_cast<int>(d.exponent));
+            }
+            return write_double_value_at(w, decimal_to_float<double>(d)); // (without reading the token again)
+        }
+        return write_double_value_at(w, static_cast<double>(float_value<number_float_t>(m_doc, n)));
+    }
+
+    /// n bytes of text at w
+    static char* write_text_at(char* w, const char* text, std::size_t n) noexcept
+    {
+        std::memcpy(w, text, n);
+        return w + n;
+    }
+
+    /// a double as dump() writes it, at w (64 bytes of room)
+    static char* write_double_value_at(char* w, double x)
+    {
+        // (from the bits: without the checks of to_chars())
+        std::uint64_t bits = 0;
+        std::memcpy(&bits, &x, sizeof(bits));
+        if (NLOHMANN_VIEW_UNLIKELY((bits & 0x7FF0000000000000u) == 0x7FF0000000000000u))
+        {
+            return write_text_at(w, "null", 4);
+        }
+        *w = '-';
+        w += bits >> 63u;
+        bits &= ~(std::uint64_t{1} << 63u);
+        if (bits == 0)
+        {
+            return write_text_at(w, "0.0", 3);
+        }
+        return ::nlohmann::detail::dtoa_impl::write_shortest(w, ::nlohmann::detail::zmij::to_shortest(bits));
+    }
+
     /// as serializer::dump_float()
     void write_float(number_float_t x)
     {
@@ -327,7 +859,9 @@ class view_serializer
         m_out.put('"');
     }
 
-    /// as serializer::dump_escaped() for valid UTF-8 (the view has no other)
+    /// as serializer::dump_escaped(); strings of a document are valid UTF-8,
+    /// except in a damaged image loaded with image_check::bounds, for which
+    /// this throws what basic_json::dump() throws for the string
     template<bool EnsureAscii>
     void write_escaped(const unsigned char* s, std::size_t n)
     {
@@ -351,12 +885,13 @@ class view_serializer
             }
             std::uint32_t codepoint = s[i];
             std::size_t len = 1;
-            if (codepoint >= 0xC0)
+            if (codepoint >= 0x80)
             {
-                len = 2;
-                if (codepoint >= 0xE0)
+                len = validate_one_utf8(s + i, n - i);
+                if (NLOHMANN_VIEW_UNLIKELY(len == 0))
                 {
-                    len = codepoint >= 0xF0 ? 4 : 3;
+                    invalid_utf8(s, n);
+                    return;
                 }
                 codepoint &= 0xFFu >> (len + 1);
                 for (std::size_t k = 1; k < len; ++k)
@@ -369,6 +904,13 @@ class view_serializer
         }
     }
 
+    /// throw what basic_json::dump() throws for a string that is not valid UTF-8
+    NLOHMANN_VIEW_NOINLINE static void invalid_utf8(const unsigned char* s, std::size_t n)
+    {
+        const string_t dumped = BasicJsonType(string_t(reinterpret_cast<const char*>(s), n)).dump(); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+        static_cast<void>(dumped);
+    }
+
     template<bool EnsureAscii>
     void write_codepoint(std::uint32_t codepoint, const unsigned char* bytes, std::size_t len)
     {

include/nlohmann/json_view.hpp

@@ -29,7 +29,7 @@
 #include <algorithm> // all_of, min
 #include <array> // array
 #include <cstddef> // nullptr_t, size_t // IWYU pragma: keep
-#include <cstdint> // uint32_t
+#include <cstdint> // uint8_t, uint32_t
 #include <cstring> // memcpy, strlen
 #include <istream> // istream // IWYU pragma: keep
 #include <iterator> // distance, input_iterator_tag, iterator_traits
@@ -71,6 +71,7 @@
 #include <nlohmann/detail/view/document_data.hpp>
 #include <nlohmann/detail/view/edit.hpp>
 #include <nlohmann/detail/view/errors.hpp>
+#include <nlohmann/detail/view/image.hpp>
 #include <nlohmann/detail/view/input.hpp>
 #include <nlohmann/detail/view/iterator.hpp>
 #include <nlohmann/detail/view/lookup.hpp>
@@ -622,8 +623,10 @@ class basic_json_view
         style.indent_char = indent_char;
         style.ensure_ascii = ensure_ascii;
         style.source_numbers = numbers == number_format::source;
-        // the compact text is about as long as the source text of the value
-        const std::size_t estimate = source_extent() + (style.pretty ? source_extent() / 2 : 0) + 64;
+        // the compact text is about as long as the source text of the value;
+        // the compact writer keeps 64 bytes of slack, so that it does not grow
+        // the buffer just before the end
+        const std::size_t estimate = source_extent() + (style.pretty ? source_extent() / 2 : 0) + 160;
         detail::view::view_serializer<BasicJsonType, Editable>(*m_doc, out, estimate, style).dump(m_node);
         return out;
     }
@@ -1028,7 +1031,7 @@ class basic_json_document
     /// whether the document holds its own copy of the text
     bool owns_source() const noexcept
     {
-        return m_data && !m_data->owned.empty() && m_data->src == m_data->owned.data();
+        return m_data && ((!m_data->owned.empty() && m_data->src == m_data->owned.data()) || !m_data->owned_image.empty());
     }
 
     /// number of index nodes (values plus object keys)
@@ -1037,7 +1040,7 @@ class basic_json_document
         return m_data ? m_data->node_count : 0;
     }
 
-    /// bytes held by the document (index, decoded strings, owned text)
+    /// bytes held by the document (index, decoded strings, and owned text or image)
     std::size_t memory_usage() const noexcept
     {
         if (!m_data)
@@ -1046,7 +1049,7 @@ class basic_json_document
         }
         return sizeof(document_data) + (m_data->inline_capacity * sizeof(detail::view::node))
                + (m_data->nodes != m_data->inline_nodes ? m_data->node_capacity * sizeof(detail::view::node) : 0)
-               + m_data->arena.capacity() + m_data->owned.capacity()
+               + m_data->arena.capacity() + m_data->owned.capacity() + m_data->owned_image.capacity()
                + (m_data->indexes.capacity() * sizeof(document_data::object_index)) + (m_data->index_slots.capacity() * sizeof(std::uint32_t))
                + (m_data->large_objects.capacity() * sizeof(std::uint32_t))
                + (m_data->edits != nullptr ? m_data->edits->bytes : 0);
@@ -1066,8 +1069,10 @@ class basic_json_document
 
         // allocate everything first, so that an exception leaves the document
         // unchanged
+        // (the decoded strings of a loaded image stay in the image)
+        const bool arena_in_use = d.base[1] == d.arena.data();
         const bool shrink_arena = d.arena.capacity() > d.arena.size();
-        std::vector<char> arena(shrink_arena ? d.arena : std::vector<char>());
+        std::vector<char> arena(shrink_arena && arena_in_use ? d.arena : std::vector<char>());
         // (edits link to the nodes of the index, which then stays in place)
         const bool shrink_nodes = d.nodes != d.inline_nodes && d.node_count != d.node_capacity && d.edits == nullptr;
         const bool into_header = d.node_count <= d.inline_capacity;
@@ -1088,7 +1093,10 @@ class basic_json_document
         if (shrink_arena)
         {
             d.arena.swap(arena);
-            d.base[1] = d.arena.data();
+            if (arena_in_use)
+            {
+                d.base[1] = d.arena.data();
+            }
         }
         if (d.indexes.capacity() > d.indexes.size())
         {
@@ -1100,6 +1108,55 @@ class basic_json_document
         }
     }
 
+    ////////////
+    // images //
+    ////////////
+
+    /// how load() checks an image (full, bounds, or none)
+    using image_check = detail::view::image_check;
+
+    /// The document as an image that load() reads without parsing: the node
+    /// index, the text, and the decoded strings. An edited document is
+    /// written in its current state (floats that are not finite become null,
+    /// as in dump()).
+    std::vector<std::uint8_t> save() const
+    {
+        if (NLOHMANN_VIEW_UNLIKELY(!m_data || m_data->discarded))
+        {
+            detail::view::throw_type_error(320, "cannot save a discarded json_document");
+        }
+        return detail::view::save_image(*m_data);
+    }
+
+    /// Read an image written by save(). The image is borrowed: it must stay
+    /// alive and unchanged while the document is used.
+    NLOHMANN_VIEW_NODISCARD
+    static basic_json_document load(const std::uint8_t* image, std::size_t size, const image_check check = image_check::full)
+    {
+        basic_json_document d;
+        d.ensure_data(nullptr, 0);
+        detail::view::load_image(*d.m_data, image, size, check);
+        return d;
+    }
+
+    /// read an image (borrowed)
+    NLOHMANN_VIEW_NODISCARD
+    static basic_json_document load(const std::vector<std::uint8_t>& image, const image_check check = image_check::full)
+    {
+        return load(image.data(), image.size(), check);
+    }
+
+    /// read an image and keep it (no copy)
+    NLOHMANN_VIEW_NODISCARD
+    static basic_json_document load(std::vector<std::uint8_t>&& image, const image_check check = image_check::full)
+    {
+        basic_json_document d;
+        d.ensure_data(nullptr, 0);
+        d.m_data->owned_image = std::move(image);
+        detail::view::load_image(*d.m_data, d.m_data->owned_image.data(), d.m_data->owned_image.size(), check);
+        return d;
+    }
+
     ///////////
     // edits //
     ///////////
@@ -1334,6 +1391,7 @@ class basic_json_document
         {
             d.owned.clear();
         }
+        d.owned_image.clear();
         d.src = src;
         d.size = size;
         d.node_count = 0;
@@ -1358,6 +1416,7 @@ class basic_json_document
         {
             d.base[0] = d.src;
             d.base[1] = d.arena.data();
+            d.arena_size = d.arena.size();
             detail::view::build_object_indexes(d);
             std::vector<std::uint32_t>().swap(d.large_objects); // (only needed while parsing)
             d.discarded = false;

tests/CMakeLists.txt

@@ -2,12 +2,14 @@ cmake_minimum_required(VERSION 3.13...4.0)
 
 option(JSON_Valgrind    "Execute test suite with Valgrind." OFF)
 option(JSON_FastTests   "Skip expensive/slow tests." OFF)
+option(JSON_TestUnityBuild "Build the unit tests in batches of several files per executable to speed up compilation." ON)
 option(JSON_TestSimdutf "Build the unit tests against the simdutf UTF-8 validation backend." OFF)
 
 set(JSON_SIMDUTF_VERSION 9.1.0 CACHE STRING "The simdutf version used by JSON_TestSimdutf.")
 
 set(JSON_32bitTest     AUTO CACHE STRING "Enable the 32bit unit test (ON/OFF/AUTO/ONLY).")
 set(JSON_TestStandards "" CACHE STRING "The list of standards to test explicitly.")
+set(JSON_TestUnityBatchSize 8 CACHE STRING "The number of unit test files compiled into one executable with JSON_TestUnityBuild.")
 set(JSON_TestShard     "" CACHE STRING "Build only a part of the unit tests, given as <index>/<count> (e.g. 0/2), to split them across CI jobs with a time limit.")
 
 # using an env var, since this will also affect targets executing cmake (such as "ci_test_compiler_default")
@@ -20,6 +22,11 @@ endif()
 
 include(test)
 
+# MinGW's GNU ld hits section and relocation limits with the large merged translation units
+if(MINGW)
+    set(JSON_TestUnityBuild OFF)
+endif()
+
 #############################################################################
 # override standard support
 #############################################################################
@@ -326,9 +333,28 @@ if(NOT "${JSON_TestShard}" STREQUAL "")
     message(STATUS "Test shard ${JSON_TestShard}: ${shard_file_count} of ${test_file_count} unit test files")
 endif()
 
-foreach(file ${files})
-    json_test_add_test_for(${file} MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force})
-endforeach()
+# Explicit groups of test files (file names without "unit-" and ".cpp") for
+# JSON_TestUnityBuild: all files of a group are compiled into one translation
+# unit per C++ standard, so that related tests share their template
+# instantiations (all binary formats instantiate the same readers and
+# writers). Only add a group where this was measured to pay off: grouping
+# files that share little just creates one slow translation unit. Files in no
+# group are batched alphabetically by JSON_TestUnityBatchSize.
+set(json_test_unity_groups binary)
+set(json_test_unity_group_binary binary_formats binary_utf8_error_handler binary_writer_sinks bjdata bon8 bon8-cpp17 bson byte_container_with_subtype cbor msgpack msgpack-cpp17 ubjson)
+
+if(JSON_TestUnityBuild)
+    message(STATUS "Building the unit tests in batches of ${JSON_TestUnityBatchSize} files (JSON_TestUnityBuild)")
+    json_test_add_unity_tests(FILES ${files} MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
+        BATCH_SIZE ${JSON_TestUnityBatchSize}
+        GROUPS ${json_test_unity_groups}
+        VARIANT_FILES src/unit-comparison.cpp src/unit-comparison-cpp20.cpp src/unit-class_parser.cpp src/unit-diagnostic-positions.cpp
+    )
+else()
+    foreach(file ${files})
+        json_test_add_test_for(${file} MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force})
+    endforeach()
+endif()
 
 # tests/src/unit-no-macro-leak.cpp #include-s the generated leak-check file,
 # so its test targets must be built after generate_hedley_undef_checks.
@@ -354,6 +380,13 @@ json_test_add_test_for(src/unit-comparison.cpp
     NAME test-comparison_legacy
     MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
 )
+json_test_set_test_options(test-comparison_legacy-cpp20
+    COMPILE_DEFINITIONS JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1
+)
+json_test_add_test_for(src/unit-comparison-cpp20.cpp
+    NAME test-comparison_legacy-cpp20
+    MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
+)
 
 # test the parser again with JSON_DIAGNOSTIC_POSITIONS enabled
 json_test_set_test_options(test-class_parser_diagnostic_positions

tests/Makefile

@@ -10,7 +10,7 @@ CXXFLAGS += -std=c++11
 CPPFLAGS += -I ../single_include
 
 FUZZER_ENGINE = src/fuzzer-driver_afl.cpp
-FUZZERS = parse_afl_fuzzer parse_bson_fuzzer parse_cbor_fuzzer parse_msgpack_fuzzer parse_ubjson_fuzzer parse_bjdata_fuzzer parse_bon8_fuzzer parse_json_view_fuzzer
+FUZZERS = parse_afl_fuzzer parse_bson_fuzzer parse_cbor_fuzzer parse_msgpack_fuzzer parse_ubjson_fuzzer parse_bjdata_fuzzer parse_bon8_fuzzer parse_json_view_fuzzer json_view_image_fuzzer
 fuzzers: $(FUZZERS)
 
 parse_afl_fuzzer:
@@ -19,6 +19,9 @@ parse_afl_fuzzer:
 parse_json_view_fuzzer:
 	$(CXX) $(CXXFLAGS) $(CPPFLAGS) $(FUZZER_ENGINE) src/fuzzer-parse_json_view.cpp -o $@
 
+json_view_image_fuzzer:
+	$(CXX) $(CXXFLAGS) $(CPPFLAGS) $(FUZZER_ENGINE) src/fuzzer-json_view_image.cpp -o $@
+
 parse_bson_fuzzer:
 	$(CXX) $(CXXFLAGS) $(CPPFLAGS) $(FUZZER_ENGINE) src/fuzzer-parse_bson.cpp -o $@
 

tests/README.md

@@ -0,0 +1,92 @@
+# Unit tests
+
+The unit tests are in [`src/unit-*.cpp`](src) and use [doctest](https://github.com/doctest/doctest). Each file becomes
+one CTest test per C++ standard it is built for, named after the file: `src/unit-foo.cpp` becomes `test-foo_cpp11`.
+
+## Build and run
+
+```sh
+cmake -S . -B build -DJSON_BuildTests=ON
+cmake --build build -j 10
+ctest --test-dir build -j 10
+```
+
+The [CMake options](../docs/mkdocs/docs/integration/cmake.md) starting with `JSON_Test` (and `JSON_FastTests`,
+`JSON_Valgrind`) control the test build. The most relevant ones:
+
+- `JSON_TestStandards`: build every test file for the given standards, e.g., `-DJSON_TestStandards=17`. By default,
+  a file is built for C++11 and for each standard whose `JSON_HAS_CPP_<N>` macro it mentions (see below).
+- `JSON_TestUnityBuild`: compile several test files together (see below). Set it to `OFF` to get one executable per
+  test file, e.g., to debug a single test in a debugger.
+- `JSON_TestShard`: build only a part of the test files, e.g., `-DJSON_TestShard=0/2`.
+
+To run the tests of one file only, run its CTest test, e.g., `ctest --test-dir build -R test-foo_cpp11`.
+
+## How test files are built
+
+Two mechanisms keep the compile time down. Both are automatic, but they set a few rules for test files.
+
+### C++ standards
+
+A test file is built for a C++ standard beyond C++11 if its text contains `JSON_HAS_CPP_<N>`, for instance in
+`#ifdef JSON_HAS_CPP_17`, or in a comment `// JSON_HAS_CPP_17` next to code that needs a macro only defined for that
+standard, such as `JSON_HAS_FILESYSTEM`. Then the *whole file* is compiled again for that standard.
+
+So that this does not happen for the large files, tests that need C++14, C++17, or C++20 go into a separate file
+`src/unit-<name>-cpp<N>.cpp` (e.g., [`src/unit-regression3-cpp17.cpp`](src/unit-regression3-cpp17.cpp)), whose body
+is wrapped in `#ifdef JSON_HAS_CPP_<N>`. The main file `src/unit-<name>.cpp` must not mention `JSON_HAS_CPP_<N>` at
+all, not even in a comment, so it is built for C++11 only. The CI jobs `ci_test_*_cxx<N>` still build every test file
+for every standard.
+
+### Unity build
+
+With `JSON_TestUnityBuild` (`ON` by default, `OFF` with MinGW), CMake compiles several test files as one translation
+unit, so they share the template instantiations of the library. CMake generates `build/tests/unity/test-unity-*.cpp`,
+which `#include` the test files of a batch. Every test file still gets its own CTest test, which runs the batch
+executable with `--source-file=*unit-foo.cpp`, so only the test cases of that file run.
+
+CMake decides from the macros a file defines *before* including `<nlohmann/json.hpp>`:
+
+| Macros defined before the include                                                         | Built as                          |
+|-------------------------------------------------------------------------------------------|-----------------------------------|
+| none (or only `SKIP_TESTS_FOR_*` and similar macros derived from global definitions)      | batch with the other plain files  |
+| only `JSON_TESTS_PRIVATE`                                                                 | batch with the other such files   |
+| any library configuration macro (`JSON_DIAGNOSTICS`, `JSON_NO_IO`, `JSON_ASSERT`, ...)    | its own executable                |
+
+Files with test-specific build options (`json_test_set_test_options(test-foo ...)` in
+[`CMakeLists.txt`](CMakeLists.txt)) also get their own executable. The batchable files are split alphabetically into
+batches of `JSON_TestUnityBatchSize` files, except for explicit groups of related files: `json_test_unity_group_binary`
+compiles all binary-format tests together, because they share the binary readers and writers. Only add a group if it
+measurably pays off; files that share little just make one slow translation unit.
+
+## Rules for test files
+
+Because the files of a batch share one translation unit, a test file must not affect the files that follow it:
+
+1. **Give file-scope helpers file-specific names.** An anonymous namespace does not help, as the batch is one
+   translation unit. Use a name such as `my_allocator_2982`, or wrap the helpers in a namespace named after the file
+   (e.g., `namespace unit_comparison_detail`). Clashes show up as compile errors.
+2. **Do not `#undef` library macros**, such as `JSON_HAS_CPP_17`, at the end of a file: this silently disables the tests
+   that depend on them in the later files of the batch. `#undef` macros you define yourself after including the library.
+3. **Put version-dependent tests into `unit-<name>-cpp<N>.cpp`** (see [C++ standards](#c-standards)).
+4. **Define test cases in the `.cpp` file itself.** The batch executable selects the tests of a file by its name, so a
+   `TEST_CASE` in a shared header is not run. The only exception is the test case in
+   [`src/make_test_data_available.hpp`](src/make_test_data_available.hpp), which CMake handles explicitly.
+
+To check a new file for clashes with all others at once, build with `-DJSON_TestUnityBatchSize=1000`, which puts all
+batchable files of a kind into one translation unit.
+
+## Where to add tests
+
+Tests are structured along the features of the library. Usually, an existing file is the right place:
+
+- For a bug fix, add a section referencing the issue to [`src/unit-regression3.cpp`](src/unit-regression3.cpp), or to
+  [`src/unit-regression3-cpp17.cpp`](src/unit-regression3-cpp17.cpp) /
+  [`src/unit-regression3-cpp20.cpp`](src/unit-regression3-cpp20.cpp) if the test needs C++17 / C++20.
+  [`src/unit-regression2.cpp`](src/unit-regression2.cpp) holds older tests and should not grow further.
+- When testing exceptions, use `CHECK_THROWS_WITH_AS`, which also checks the `what()` message.
+
+A new file `src/unit-<name>.cpp` is picked up automatically when CMake runs again; no change to `CMakeLists.txt` is
+needed.
+
+See also [`fuzzing.md`](fuzzing.md) for fuzz testing.

tests/fuzzing.md

@@ -12,6 +12,12 @@ produces, and that a rejected input makes both parsers throw with an identical `
 are taken from the low bits of the first input byte (the byte stays part of the text). It takes plain JSON text, so it
 reuses the `corpus_json` corpus rather than a format of its own.
 
+`json_view_image_fuzzer` (`tests/src/fuzzer-json_view_image.cpp`) tests the images of `json_document` (`save()` and
+`load()`). It uses each input twice: as an image, which `load()` must either reject with `parse_error.116` or read
+safely (with `image_check::full`, the document must also serialize to the JSON it reads as), and as a JSON text, whose
+image must load and serialize to the same text. A corpus of images can be made from JSON files with a small program
+that calls `json_document::parse(text).save()`; plain JSON files work as well.
+
 ## What the fuzzers check
 
 Each fuzzer driver (`tests/src/fuzzer-parse_*.cpp`) parses its input twice: once with `allow_exceptions = false` and

tests/src/fuzzer-json_view_image.cpp

@@ -0,0 +1,95 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+/*
+This file implements a test of json_document images suitable for fuzz
+testing. The input is used twice:
+
+- as an image: json_document::load() with image_check::full must either throw
+  a parse_error or yield a document that serializes to the JSON text it reads
+  as; with image_check::bounds, reading and serializing must be safe (checked
+  by the sanitizers), and serializing may only throw type_error.316
+- as a JSON text: if json_document::parse() accepts it, the image of the
+  document must load (with every check) and serialize to the same text
+
+The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
+drivers.
+*/
+
+#include <cassert>
+#include <cstdint>
+#include <string>
+#include <vector>
+#include <nlohmann/json.hpp>
+#include <nlohmann/json_view.hpp>
+
+// the checks below are assertions; NDEBUG would compile them away
+#ifdef NDEBUG
+    #error "the fuzzer drivers must be built without NDEBUG"
+#endif
+
+using json = nlohmann::json;
+using json_document = nlohmann::json_document;
+using image_check = json_document::image_check;
+
+// see http://llvm.org/docs/LibFuzzer.html
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
+{
+    // the input as an image
+    for (const image_check check :
+            {
+                image_check::full, image_check::bounds
+            })
+    {
+        json_document d;
+        try
+        {
+            d = json_document::load(data, size, check);
+        }
+        catch (const json::parse_error& e)
+        {
+            assert(e.id == 116);
+            continue;
+        }
+        std::string dumped;
+        try
+        {
+            dumped = d.root().dump();
+        }
+        catch (const json::type_error& e)
+        {
+            // invalid UTF-8 can only pass the bounds check
+            assert(check == image_check::bounds && e.id == 316);
+            continue;
+        }
+        const json j = d.root().materialize();
+        if (check == image_check::full)
+        {
+            assert(json::parse(dumped) == j);
+            // an image of the loaded document is the input
+            assert(d.save() == std::vector<std::uint8_t>(data, data + size));
+        }
+    }
+
+    // the input as a JSON text
+    const std::string text(reinterpret_cast<const char*>(data), size); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+    const json_document parsed = json_document::parse(text, false);
+    if (!parsed.is_discarded())
+    {
+        const std::vector<std::uint8_t> image = parsed.save();
+        for (const image_check check :
+                {
+                    image_check::full, image_check::bounds, image_check::none
+                })
+        {
+            const json_document loaded = json_document::load(image, check);
+            assert(loaded.root().dump() == parsed.root().dump());
+        }
+    }
+    return 0;
+}

tests/src/json_view_test_helpers.hpp

@@ -15,6 +15,7 @@
 #include <nlohmann/json_view.hpp>
 
 #include <algorithm> // any_of
+#include <functional> // function
 #include <random> // mt19937
 #include <string> // string
 
@@ -86,4 +87,20 @@ inline bool has_duplicate_keys(const nlohmann::ordered_json_view& v)
         return e.is_structured() && has_duplicate_keys(e);
     });
 }
+
+#if !defined(JSON_NOEXCEPTION)
+// the exception a call throws, or "" if it throws none
+inline std::string exception_of_call(const std::function<void()>& f)
+{
+    try
+    {
+        f();
+    }
+    catch (const nlohmann::json::exception& e)
+    {
+        return e.what();
+    }
+    return "";
+}
+#endif
 } // namespace json_view_test

tests/src/unit-allocator-cpp17.cpp

@@ -0,0 +1,98 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-allocator.cpp (std::string_view key
+// lookup with a failing allocator). It is kept in a separate translation unit so the (much
+// larger) unit-allocator.cpp is built for C++11 only and not rebuilt for every C++
+// standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#ifdef JSON_HAS_CPP_17
+#include <memory>
+#include <new>
+#include <string_view>
+#include <utility>
+
+namespace
+{
+bool next_construct_fails_cpp17 = false;
+bool next_destroy_fails_cpp17 = false;
+bool next_deallocate_fails_cpp17 = false;
+
+template<class T>
+struct my_allocator_cpp17 : std::allocator<T>
+{
+    using std::allocator<T>::allocator;
+
+    template<class... Args>
+    void construct(T* p, Args&& ... args)
+    {
+        if (next_construct_fails_cpp17)
+        {
+            next_construct_fails_cpp17 = false;
+            throw std::bad_alloc();
+        }
+
+        ::new (reinterpret_cast<void*>(p)) T(std::forward<Args>(args)...);
+    }
+
+    void deallocate(T* p, std::size_t n)
+    {
+        if (next_deallocate_fails_cpp17)
+        {
+            next_deallocate_fails_cpp17 = false;
+            throw std::bad_alloc();
+        }
+
+        std::allocator<T>::deallocate(p, n);
+    }
+
+    void destroy(T* p)
+    {
+        if (next_destroy_fails_cpp17)
+        {
+            next_destroy_fails_cpp17 = false;
+            throw std::bad_alloc();
+        }
+
+        static_cast<void>(p); // fix MSVC's C4100 warning
+        p->~T();
+    }
+
+    template <class U>
+    struct rebind
+    {
+        using other = my_allocator_cpp17<U>;
+    };
+};
+} // namespace
+
+// the no-exceptions CI job skips every CHECK_THROWS_AS, which would leave
+// next_construct_fails_cpp17 set for the next allocation outside a check
+#if !defined(JSON_NOEXCEPTION)
+TEST_CASE("a failed allocation leaves the value unchanged (C++17)")
+{
+    // create JSON type using the throwing allocator
+    using my_json = nlohmann::json::with_allocator_t<my_allocator_cpp17>;
+
+    SECTION("turning a null value into an array or object")
+    {
+        my_json j;
+
+        next_construct_fails_cpp17 = true;
+        CHECK_THROWS_AS(j[std::string_view("key")], std::bad_alloc&);
+        CHECK(j.is_null());
+
+        next_construct_fails_cpp17 = false;
+    }
+}
+#endif
+#endif

tests/src/unit-allocator-cpp20.cpp

@@ -0,0 +1,115 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++20-only part of unit-allocator.cpp (creating a value from a
+// std::ranges view with a failing allocator). It is kept in a separate translation unit so
+// the (much larger) unit-allocator.cpp is built for C++11 only and not rebuilt for every
+// C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#ifdef JSON_HAS_CPP_20
+#if JSON_HAS_RANGES
+    #include <ranges>
+#endif
+
+#include <memory>
+#include <new>
+#include <utility>
+#include <vector>
+
+namespace
+{
+bool next_construct_fails_cpp20 = false;
+bool next_destroy_fails_cpp20 = false;
+bool next_deallocate_fails_cpp20 = false;
+
+template<class T>
+struct my_allocator_cpp20 : std::allocator<T>
+{
+    using std::allocator<T>::allocator;
+
+    template<class... Args>
+    void construct(T* p, Args&& ... args)
+    {
+        if (next_construct_fails_cpp20)
+        {
+            next_construct_fails_cpp20 = false;
+            throw std::bad_alloc();
+        }
+
+        ::new (reinterpret_cast<void*>(p)) T(std::forward<Args>(args)...);
+    }
+
+    void deallocate(T* p, std::size_t n)
+    {
+        if (next_deallocate_fails_cpp20)
+        {
+            next_deallocate_fails_cpp20 = false;
+            throw std::bad_alloc();
+        }
+
+        std::allocator<T>::deallocate(p, n);
+    }
+
+    void destroy(T* p)
+    {
+        if (next_destroy_fails_cpp20)
+        {
+            next_destroy_fails_cpp20 = false;
+            throw std::bad_alloc();
+        }
+
+        static_cast<void>(p); // fix MSVC's C4100 warning
+        p->~T();
+    }
+
+    template <class U>
+    struct rebind
+    {
+        using other = my_allocator_cpp20<U>;
+    };
+};
+} // namespace
+
+// the no-exceptions CI job skips every CHECK_THROWS_AS, which would leave
+// next_construct_fails_cpp20 set for the next allocation outside a check
+#if !defined(JSON_NOEXCEPTION)
+TEST_CASE("a failed allocation leaves the value unchanged (C++20)")
+{
+    // create JSON type using the throwing allocator
+    using my_json = nlohmann::json::with_allocator_t<my_allocator_cpp20>;
+
+    // With iterator debugging, VS 2015's containers construct a proxy with the
+    // allocator in constructors that cannot report its failure, so a failing
+    // allocator crashes this section there (SIGSEGV with VS 2015 Debug x86).
+#if !(defined(_MSC_VER) && _MSC_VER < 1910 && defined(_ITERATOR_DEBUG_LEVEL) && _ITERATOR_DEBUG_LEVEL > 0)
+    SECTION("converting into an existing value")
+    {
+        // to_json replaces the value it is given; the old one must survive a
+        // failed creation of the new one
+        my_json j = "old";
+
+#if JSON_HAS_RANGES && !defined(__MINGW32__)
+        const std::vector<int> numbers = {1, 2};
+        next_construct_fails_cpp20 = true;
+        CHECK_THROWS_AS(nlohmann::to_json(j, numbers | std::views::filter([](int /*unused*/)
+        {
+            return true;
+        })), std::bad_alloc&);
+        CHECK(j == "old");
+#endif
+
+        next_construct_fails_cpp20 = false;
+    }
+#endif
+}
+#endif
+#endif

tests/src/unit-allocator.cpp

@@ -13,9 +13,6 @@
 using nlohmann::json;
 
 #include <valarray>
-#if JSON_HAS_RANGES
-    #include <ranges>
-#endif
 
 namespace
 {
@@ -684,12 +681,6 @@ TEST_CASE("a failed allocation leaves the value unchanged")
         CHECK_THROWS_AS(j["key"], std::bad_alloc&);
         CHECK(j.is_null());
 
-#ifdef JSON_HAS_CPP_17
-        next_construct_fails = true;
-        CHECK_THROWS_AS(j[std::string_view("key")], std::bad_alloc&);
-        CHECK(j.is_null());
-#endif
-
         next_construct_fails = true;
         CHECK_THROWS_AS(j.push_back(my_json(1)), std::bad_alloc&);
         CHECK(j.is_null());
@@ -787,16 +778,6 @@ TEST_CASE("a failed allocation leaves the value unchanged")
         CHECK_THROWS_AS(nlohmann::to_json(j, std::valarray<int> {1, 2}), std::bad_alloc&);
         CHECK(j == "old");
 
-#if JSON_HAS_RANGES && !defined(__MINGW32__)
-        const std::vector<int> numbers = {1, 2};
-        next_construct_fails = true;
-        CHECK_THROWS_AS(nlohmann::to_json(j, numbers | std::views::filter([](int /*unused*/)
-        {
-            return true;
-        })), std::bad_alloc&);
-        CHECK(j == "old");
-#endif
-
         next_construct_fails = false;
         nlohmann::to_json(j, std::vector<int> {1, 2});
         CHECK(j == my_json({1, 2}));

tests/src/unit-binary_writer_sinks.cpp

@@ -11,8 +11,10 @@
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
+#include <array>
 #include <cstdint>
 #include <limits>
+#include <sstream>
 #include <string>
 #include <vector>
 
@@ -35,6 +37,17 @@ std::vector<json> test_values()
         big_object[std::to_string(i)] = i;
     }
 
+    // large strings and binary values between small ones: a write too large
+    // for the space the vector sink has ahead takes a different path than a
+    // small one, and the small writes after it must still land behind it
+    json large_payloads = json::array();
+    for (int i = 0; i < 50; ++i)
+    {
+        large_payloads.push_back(std::string(300, static_cast<char>('a' + (i % 26))));
+        large_payloads.push_back(i);
+        large_payloads.push_back(json::binary(std::vector<std::uint8_t>(static_cast<std::size_t>(100 * i), static_cast<std::uint8_t>(i))));
+    }
+
     return
     {
         json(nullptr), json(true), json(false),
@@ -46,10 +59,23 @@ std::vector<json> test_values()
         json::array(), json::object(),
         json::array({1, 2, 3}), json({{"a", 1}, {"b", nullptr}}),
         json({{"nested", {{"deep", json::array({1, "two", 3.0, nullptr})}}}}),
-        big_array, big_object
+        big_array, big_object, large_payloads,
+        json::binary(std::vector<std::uint8_t>(5000, 0x42))
     };
 }
 
+// explicit char -> uint8_t conversion (the iterator-range constructor converts implicitly, which -fsanitize=integer flags)
+std::vector<std::uint8_t> to_bytes(const std::string& str)
+{
+    std::vector<std::uint8_t> result;
+    result.reserve(str.size());
+    for (const char c : str)
+    {
+        result.push_back(static_cast<std::uint8_t>(c));
+    }
+    return result;
+}
+
 // BON8 has no integers above the int64 range, so to_bon8() rejects them
 bool bon8_representable(const json& j)
 {
@@ -170,6 +196,258 @@ TEST_CASE("binary writer output sinks")
     }
 }
 
+namespace
+{
+
+// the CBOR head of an integer, encoded independently of binary_writer
+void reference_cbor_head(std::vector<std::uint8_t>& out, const std::uint8_t major_type, const std::uint64_t argument)
+{
+    if (argument <= 23)
+    {
+        out.push_back(static_cast<std::uint8_t>(major_type + argument));
+        return;
+    }
+
+    int length = 8;
+    std::uint8_t additional = 27;
+    if (argument <= 0xFF)
+    {
+        length = 1;
+        additional = 24;
+    }
+    else if (argument <= 0xFFFF)
+    {
+        length = 2;
+        additional = 25;
+    }
+    else if (argument <= 0xFFFFFFFF)
+    {
+        length = 4;
+        additional = 26;
+    }
+
+    out.push_back(static_cast<std::uint8_t>(major_type + additional));
+    for (int i = length - 1; i >= 0; --i)
+    {
+        out.push_back(static_cast<std::uint8_t>(argument >> (8 * i)));
+    }
+}
+
+// the CBOR encoding of an integer value
+void reference_cbor_integer(std::vector<std::uint8_t>& out, const json& j)
+{
+    if (j.is_number_unsigned())
+    {
+        reference_cbor_head(out, 0x00, j.get<std::uint64_t>());
+        return;
+    }
+
+    const auto n = j.get<std::int64_t>();
+    if (n >= 0)
+    {
+        reference_cbor_head(out, 0x00, static_cast<std::uint64_t>(n));
+    }
+    else
+    {
+        reference_cbor_head(out, 0x20, static_cast<std::uint64_t>(-1 - n));
+    }
+}
+
+// the MessagePack encoding of an integer value, encoded independently of
+// binary_writer
+void reference_msgpack_integer(std::vector<std::uint8_t>& out, const json& j)
+{
+    const auto put = [&out](const std::uint8_t marker, const std::uint64_t bits, const int length)
+    {
+        out.push_back(marker);
+        for (int i = length - 1; i >= 0; --i)
+        {
+            out.push_back(static_cast<std::uint8_t>(bits >> (8 * i)));
+        }
+    };
+
+    if (j.is_number_unsigned() || j.get<std::int64_t>() >= 0)
+    {
+        const auto u = j.get<std::uint64_t>();
+        if (u < 128)
+        {
+            out.push_back(static_cast<std::uint8_t>(u));
+        }
+        else if (u <= 0xFF)
+        {
+            put(0xCC, u, 1);
+        }
+        else if (u <= 0xFFFF)
+        {
+            put(0xCD, u, 2);
+        }
+        else if (u <= 0xFFFFFFFF)
+        {
+            put(0xCE, u, 4);
+        }
+        else
+        {
+            put(0xCF, u, 8);
+        }
+        return;
+    }
+
+    const auto n = j.get<std::int64_t>();
+    const auto bits = static_cast<std::uint64_t>(n);
+    if (n >= -32)
+    {
+        out.push_back(static_cast<std::uint8_t>(bits));
+    }
+    else if (n >= -128)
+    {
+        put(0xD0, bits, 1);
+    }
+    else if (n >= -32768)
+    {
+        put(0xD1, bits, 2);
+    }
+    else if (n >= -2147483648LL)
+    {
+        put(0xD2, bits, 4);
+    }
+    else
+    {
+        put(0xD3, bits, 8);
+    }
+}
+
+// integers at every boundary of the CBOR and MessagePack sizes, signed and
+// unsigned
+std::vector<json> boundary_integers()
+{
+    std::vector<json> result;
+    const std::array<std::uint64_t, 38> boundaries =
+    {
+        {
+            0, 1, 22, 23, 24, 25, 31, 32, 33, 126, 127, 128, 129, 254, 255, 256, 257,
+            32766, 32767, 32768, 32769, 65534, 65535, 65536, 65537,
+            2147483646ULL, 2147483647ULL, 2147483648ULL, 2147483649ULL,
+            4294967294ULL, 4294967295ULL, 4294967296ULL, 4294967297ULL,
+            9223372036854775806ULL, 9223372036854775807ULL, 9223372036854775808ULL,
+            18446744073709551614ULL, 18446744073709551615ULL
+        }
+    };
+    for (const std::uint64_t u : boundaries)
+    {
+        result.emplace_back(u);
+        if (u <= static_cast<std::uint64_t>((std::numeric_limits<std::int64_t>::max)()))
+        {
+            const auto n = static_cast<std::int64_t>(u);
+            result.emplace_back(n);
+            result.emplace_back(-n);
+            result.emplace_back(-n - 1);
+        }
+    }
+    return result;
+}
+
+} // namespace
+
+// Arrays of integers are written in runs straight into space reserved from the
+// sink, with branch-free encodings; check them against independent encoders for
+// runs shorter and longer than one reservation, broken up by other values, and
+// through each sink.
+TEST_CASE("CBOR and MessagePack integer arrays")
+{
+    const std::vector<json> integers = boundary_integers();
+
+    for (const std::size_t length :
+            {
+                std::size_t{1}, std::size_t{27}, std::size_t{28}, std::size_t{29}, std::size_t{57}, std::size_t{1000}
+            })
+    {
+        for (const std::size_t break_every :
+                {
+                    std::size_t{0}, std::size_t{1}, std::size_t{7}, std::size_t{28}
+                })
+        {
+            CAPTURE(length)
+            CAPTURE(break_every)
+
+            json array = json::array();
+            std::vector<std::uint8_t> expected_cbor;
+            std::vector<std::uint8_t> expected_msgpack;
+            for (std::size_t i = 0; i < length; ++i)
+            {
+                if (break_every != 0 && i % (break_every + 1) == break_every)
+                {
+                    array.push_back("x");
+                    expected_cbor.push_back(0x61);
+                    expected_cbor.push_back('x');
+                    expected_msgpack.push_back(0xA1);
+                    expected_msgpack.push_back('x');
+                }
+                else
+                {
+                    const json& value = integers[(i * 7) % integers.size()];
+                    array.push_back(value);
+                    reference_cbor_integer(expected_cbor, value);
+                    reference_msgpack_integer(expected_msgpack, value);
+                }
+            }
+
+            std::vector<std::uint8_t> head;
+            reference_cbor_head(head, 0x80, length);
+            expected_cbor.insert(expected_cbor.begin(), head.begin(), head.end());
+
+            head.clear();
+            if (length <= 15)
+            {
+                head.push_back(static_cast<std::uint8_t>(0x90 + length));
+            }
+            else
+            {
+                head = {0xDC, static_cast<std::uint8_t>(length >> 8), static_cast<std::uint8_t>(length)};
+            }
+            expected_msgpack.insert(expected_msgpack.begin(), head.begin(), head.end());
+
+            CHECK(json::to_cbor(array) == expected_cbor);
+            CHECK(json::to_msgpack(array) == expected_msgpack);
+
+            std::vector<std::uint8_t> through_adapter = {0xAA};
+            json::to_cbor(array, through_adapter);
+            CHECK(std::vector<std::uint8_t>(through_adapter.begin() + 1, through_adapter.end()) == expected_cbor);
+            through_adapter = {0xAA};
+            json::to_msgpack(array, through_adapter);
+            CHECK(std::vector<std::uint8_t>(through_adapter.begin() + 1, through_adapter.end()) == expected_msgpack);
+
+            std::ostringstream stream;
+            json::to_cbor(array, stream);
+            std::string streamed = stream.str();
+            CHECK(to_bytes(streamed) == expected_cbor);
+            stream.str("");
+            json::to_msgpack(array, stream);
+            streamed = stream.str();
+            CHECK(to_bytes(streamed) == expected_msgpack);
+
+            CHECK(json::from_cbor(expected_cbor) == array);
+            CHECK(json::from_msgpack(expected_msgpack) == array);
+
+            // the same array as object members
+            const json object = {{"a", array}, {"b", array}};
+            CHECK(json::from_cbor(json::to_cbor(object)) == object);
+            CHECK(json::from_msgpack(json::to_msgpack(object)) == object);
+        }
+    }
+
+    // single values, outside an array
+    for (const auto& value : integers)
+    {
+        CAPTURE(value)
+        std::vector<std::uint8_t> expected;
+        reference_cbor_integer(expected, value);
+        CHECK(json::to_cbor(value) == expected);
+        expected.clear();
+        reference_msgpack_integer(expected, value);
+        CHECK(json::to_msgpack(value) == expected);
+    }
+}
+
 // binary_reserve_hint() is documented as a *lower* bound on the serialized size,
 // so that reserving it up front can never leave the returned vector holding
 // capacity beyond what the value actually needs.

tests/src/unit-bon8-cpp17.cpp

@@ -0,0 +1,60 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-bon8.cpp (BON8 with std::byte
+// containers). It is kept in a separate translation unit so the (much larger) unit-
+// bon8.cpp is built for C++11 only and not rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#ifdef JSON_HAS_CPP_17
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
+TEST_CASE("BON8 with std::byte")
+{
+    SECTION("vector roundtrip")
+    {
+        const json original =
+        {
+            {"name", "test"},
+            {"value", 42},
+            {"array", {1, 2, 3}}
+        };
+
+        const std::vector<uint8_t> temp = json::to_bon8(original);
+        std::vector<std::byte> bon8_data(temp.size());
+        for (size_t i = 0; i < temp.size(); ++i)
+        {
+            bon8_data[i] = std::byte(temp[i]);
+        }
+
+        json from_bytes;
+        CHECK_NOTHROW(from_bytes = json::from_bon8(bon8_data));
+        CHECK(from_bytes == original);
+    }
+
+    SECTION("empty vector")
+    {
+        const std::vector<std::byte> empty_data;
+        CHECK_THROWS_WITH_AS([&]()
+        {
+            [[maybe_unused]] auto result = json::from_bon8(empty_data);
+            return true;
+        }
+        (),
+        "[json.exception.parse_error.110] parse error at byte 1: syntax error while parsing BON8 value: unexpected end of input",
+        json::parse_error&);
+    }
+}
+
+#endif

tests/src/unit-bon8.cpp

@@ -1099,41 +1099,3 @@ TEST_CASE("issue #5648 - from_bon8(ptr, len) must read len bytes, not treat ptr
 #endif
 }
 
-#ifdef JSON_HAS_CPP_17
-TEST_CASE("BON8 with std::byte")
-{
-    SECTION("vector roundtrip")
-    {
-        const json original =
-        {
-            {"name", "test"},
-            {"value", 42},
-            {"array", {1, 2, 3}}
-        };
-
-        const std::vector<uint8_t> temp = json::to_bon8(original);
-        std::vector<std::byte> bon8_data(temp.size());
-        for (size_t i = 0; i < temp.size(); ++i)
-        {
-            bon8_data[i] = std::byte(temp[i]);
-        }
-
-        json from_bytes;
-        CHECK_NOTHROW(from_bytes = json::from_bon8(bon8_data));
-        CHECK(from_bytes == original);
-    }
-
-    SECTION("empty vector")
-    {
-        const std::vector<std::byte> empty_data;
-        CHECK_THROWS_WITH_AS([&]()
-        {
-            [[maybe_unused]] auto result = json::from_bon8(empty_data);
-            return true;
-        }
-        (),
-        "[json.exception.parse_error.110] parse error at byte 1: syntax error while parsing BON8 value: unexpected end of input",
-        json::parse_error&);
-    }
-}
-#endif

tests/src/unit-comparison-cpp20.cpp

@@ -0,0 +1,526 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++20-only part of unit-comparison.cpp (tests of operator<=> and
+// other three-way comparison specific behavior). It is kept in a separate translation unit
+// so the (much larger) unit-comparison.cpp is built for C++11 only and not rebuilt for
+// every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#ifdef JSON_HAS_CPP_20
+#if JSON_HAS_THREE_WAY_COMPARISON
+#include <cmath>
+#include <compare>
+#include <cstddef>
+#include <limits>
+#include <string>
+#include <utility>
+#include <vector>
+
+// this can be replaced with the doctest stl extension header in version 2.5
+namespace doctest
+{
+template<> struct StringMaker<std::partial_ordering>
+{
+    static String convert(const std::partial_ordering& order)
+    {
+        if (order == std::partial_ordering::less)
+        {
+            return "std::partial_ordering::less";
+        }
+        if (order == std::partial_ordering::equivalent)
+        {
+            return "std::partial_ordering::equivalent";
+        }
+        if (order == std::partial_ordering::greater)
+        {
+            return "std::partial_ordering::greater";
+        }
+        if (order == std::partial_ordering::unordered)
+        {
+            return "std::partial_ordering::unordered";
+        }
+        return "{?}";
+    }
+};
+} // namespace doctest
+
+TEST_CASE("lexicographical comparison operators (C++20)")
+{
+    constexpr auto f_ = false;
+    constexpr auto _t = true;
+    constexpr auto nan = std::numeric_limits<json::number_float_t>::quiet_NaN();
+    constexpr auto lt = std::partial_ordering::less;
+    constexpr auto gt = std::partial_ordering::greater;
+    constexpr auto eq = std::partial_ordering::equivalent;
+    constexpr auto un = std::partial_ordering::unordered;
+
+    INFO("using 3-way comparison");
+
+#if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
+    INFO("using legacy comparison");
+#endif
+
+    SECTION("types")
+    {
+        std::vector<json::value_t> j_types =
+        {
+            json::value_t::null,
+            json::value_t::boolean,
+            json::value_t::number_integer,
+            json::value_t::number_unsigned,
+            json::value_t::number_float,
+            json::value_t::object,
+            json::value_t::array,
+            json::value_t::string,
+            json::value_t::binary,
+            json::value_t::discarded
+        };
+
+        std::vector<std::vector<bool>> expected_lt =
+        {
+            //0   1   2   3   4   5   6   7   8   9
+            {f_, _t, _t, _t, _t, _t, _t, _t, _t, f_}, //  0
+            {f_, f_, _t, _t, _t, _t, _t, _t, _t, f_}, //  1
+            {f_, f_, f_, f_, f_, _t, _t, _t, _t, f_}, //  2
+            {f_, f_, f_, f_, f_, _t, _t, _t, _t, f_}, //  3
+            {f_, f_, f_, f_, f_, _t, _t, _t, _t, f_}, //  4
+            {f_, f_, f_, f_, f_, f_, _t, _t, _t, f_}, //  5
+            {f_, f_, f_, f_, f_, f_, f_, _t, _t, f_}, //  6
+            {f_, f_, f_, f_, f_, f_, f_, f_, _t, f_}, //  7
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  8
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  9
+        };
+
+        SECTION("comparison: less")
+        {
+            for (size_t i = 0; i < j_types.size(); ++i)
+            {
+                for (size_t j = 0; j < j_types.size(); ++j)
+                {
+                    CAPTURE(i)
+                    CAPTURE(j)
+                    // check precomputed values
+                    CHECK((j_types[i] < j_types[j]) == expected_lt[i][j]);
+                }
+            }
+        }
+
+        SECTION("comparison: 3-way")
+        {
+            // doctest runs the REQUIRE in the test case body once per leaf section; keep it
+            // here to run it as often as before the 3-way sections moved from unit-comparison.cpp
+            REQUIRE(std::isnan(nan));
+
+            std::vector<std::vector<std::partial_ordering>> expected =
+            {
+                //0   1   2   3   4   5   6   7   8   9
+                {eq, lt, lt, lt, lt, lt, lt, lt, lt, un}, //  0
+                {gt, eq, lt, lt, lt, lt, lt, lt, lt, un}, //  1
+                {gt, gt, eq, eq, eq, lt, lt, lt, lt, un}, //  2
+                {gt, gt, eq, eq, eq, lt, lt, lt, lt, un}, //  3
+                {gt, gt, eq, eq, eq, lt, lt, lt, lt, un}, //  4
+                {gt, gt, gt, gt, gt, eq, lt, lt, lt, un}, //  5
+                {gt, gt, gt, gt, gt, gt, eq, lt, lt, un}, //  6
+                {gt, gt, gt, gt, gt, gt, gt, eq, lt, un}, //  7
+                {gt, gt, gt, gt, gt, gt, gt, gt, eq, un}, //  8
+                {un, un, un, un, un, un, un, un, un, un}, //  9
+            };
+
+            // check expected partial_ordering against expected boolean
+            REQUIRE(expected.size() == expected_lt.size());
+            for (size_t i = 0; i < expected.size(); ++i)
+            {
+                REQUIRE(expected[i].size() == expected_lt[i].size());
+                for (size_t j = 0; j < expected[i].size(); ++j)
+                {
+                    CAPTURE(i)
+                    CAPTURE(j)
+                    CHECK(std::is_lt(expected[i][j]) == expected_lt[i][j]);
+                }
+            }
+
+            // check 3-way comparison against expected partial_ordering
+            REQUIRE(expected.size() == j_types.size());
+            for (size_t i = 0; i < j_types.size(); ++i)
+            {
+                REQUIRE(expected[i].size() == j_types.size());
+                for (size_t j = 0; j < j_types.size(); ++j)
+                {
+                    CAPTURE(i)
+                    CAPTURE(j)
+                    CHECK((j_types[i] <=> j_types[j]) == expected[i][j]); // *NOPAD*
+                }
+            }
+        }
+    }
+
+    SECTION("values")
+    {
+        json j_values =
+        {
+            nullptr, nullptr,                                              // 0 1
+            -17, 42,                                                       // 2 3
+            8u, 13u,                                                       // 4 5
+            3.14159, 23.42,                                                // 6 7
+            nan, nan,                                                      // 8 9
+            "foo", "bar",                                                  // 10 11
+            true, false,                                                   // 12 13
+            {1, 2, 3}, {"one", "two", "three"},                            // 14 15
+            {{"first", 1}, {"second", 2}}, {{"a", "A"}, {"b", {"B"}}},     // 16 17
+            json::binary({1, 2, 3}), json::binary({1, 2, 4}),              // 18 19
+            json(json::value_t::discarded), json(json::value_t::discarded) // 20 21
+        };
+
+        std::vector<std::vector<bool>> expected_eq =
+        {
+            //0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21
+            {_t, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  0
+            {_t, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  1
+            {f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  2
+            {f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  3
+            {f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  4
+            {f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  5
+            {f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  6
+            {f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  7
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  8
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, //  9
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, // 10
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, // 11
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_, f_}, // 12
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, f_}, // 13
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_}, // 14
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_}, // 15
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_}, // 16
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_}, // 17
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_}, // 18
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_}, // 19
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, // 20
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, // 21
+        };
+
+        std::vector<std::vector<bool>> expected_lt =
+        {
+            //0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21
+            {f_, f_, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, f_, f_}, //  0
+            {f_, f_, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, f_, f_}, //  1
+            {f_, f_, f_, _t, _t, _t, _t, _t, f_, f_, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, //  2
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, //  3
+            {f_, f_, f_, _t, f_, _t, f_, _t, f_, f_, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, //  4
+            {f_, f_, f_, _t, f_, f_, f_, _t, f_, f_, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, //  5
+            {f_, f_, f_, _t, _t, _t, f_, _t, f_, f_, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, //  6
+            {f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, //  7
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, //  8
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, //  9
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_}, // 10
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_}, // 11
+            {f_, f_, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, f_, f_, _t, _t, _t, _t, _t, _t, f_, f_}, // 12
+            {f_, f_, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, _t, f_, _t, _t, _t, _t, _t, _t, f_, f_}, // 13
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_, f_, _t, f_, f_, _t, _t, f_, f_}, // 14
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_}, // 15
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_, _t, _t, f_, f_, _t, _t, f_, f_}, // 16
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, _t, f_, f_, _t, _t, _t, f_, _t, _t, f_, f_}, // 17
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, _t, f_, f_}, // 18
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, // 19
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, // 20
+            {f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_, f_}, // 21
+        };
+
+        SECTION("signed/unsigned mixed comparison above INT64_MAX")
+        {
+            const json above_int64_max = static_cast<std::uint64_t>((std::numeric_limits<std::int64_t>::max)()) + 1ULL;
+            const json max_uint64 = (std::numeric_limits<std::uint64_t>::max)();
+            const json negative_one = -1;
+            const json one = 1;
+            const json max_int64 = (std::numeric_limits<std::int64_t>::max)();
+
+            CHECK((negative_one <=> above_int64_max) == std::partial_ordering::less); // *NOPAD*
+            CHECK((above_int64_max <=> negative_one) == std::partial_ordering::greater); // *NOPAD*
+            CHECK((negative_one <=> max_uint64) == std::partial_ordering::less); // *NOPAD*
+            CHECK((max_uint64 <=> negative_one) == std::partial_ordering::greater); // *NOPAD*
+            CHECK((one <=> above_int64_max) == std::partial_ordering::less); // *NOPAD*
+            CHECK((above_int64_max <=> one) == std::partial_ordering::greater); // *NOPAD*
+            CHECK((max_int64 <=> above_int64_max) == std::partial_ordering::less); // *NOPAD*
+            CHECK((above_int64_max <=> max_int64) == std::partial_ordering::greater); // *NOPAD*
+        }
+
+        SECTION("integer/float mixed comparison is exact")
+        {
+            // Widening the integer to a double loses precision past the
+            // mantissa, so 2^63-2 and 2^63-1 both used to compare equal to the
+            // double 2^63 while differing from each other. That makes equality
+            // intransitive and the ordering not a strict weak ordering.
+            const json below_two_63 = static_cast<std::int64_t>(9223372036854775806LL);
+            const json max_int64 = (std::numeric_limits<std::int64_t>::max)();
+            const json two_63 = 9223372036854775808.0;
+
+            // the same past the unsigned range
+            const json max_uint64 = (std::numeric_limits<std::uint64_t>::max)();
+            const json two_64 = 18446744073709551616.0;
+
+            CHECK((max_int64 <=> two_63) == std::partial_ordering::less); // *NOPAD*
+            CHECK((two_63 <=> max_int64) == std::partial_ordering::greater); // *NOPAD*
+            CHECK((below_two_63 <=> max_int64) == std::partial_ordering::less); // *NOPAD*
+            CHECK((max_uint64 <=> two_64) == std::partial_ordering::less); // *NOPAD*
+            CHECK((json(1) <=> json(1.0)) == std::partial_ordering::equivalent); // *NOPAD*
+            CHECK((json(1) <=> json(nan)) == std::partial_ordering::unordered); // *NOPAD*
+        }
+
+        SECTION("comparison: 3-way")
+        {
+            // doctest runs the REQUIRE in the test case body once per leaf section; keep it
+            // here to run it as often as before the 3-way sections moved from unit-comparison.cpp
+            REQUIRE(std::isnan(nan));
+
+            std::vector<std::vector<std::partial_ordering>> expected =
+            {
+                //0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21
+                {eq, eq, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, un, un}, //  0
+                {eq, eq, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, un, un}, //  1
+                {gt, gt, eq, lt, lt, lt, lt, lt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  2
+                {gt, gt, gt, eq, gt, gt, gt, gt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  3
+                {gt, gt, gt, lt, eq, lt, gt, lt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  4
+                {gt, gt, gt, lt, gt, eq, gt, lt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  5
+                {gt, gt, gt, lt, lt, lt, eq, lt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  6
+                {gt, gt, gt, lt, gt, gt, gt, eq, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  7
+                {gt, gt, un, un, un, un, un, un, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  8
+                {gt, gt, un, un, un, un, un, un, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  9
+                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, eq, gt, gt, gt, gt, gt, gt, gt, lt, lt, un, un}, // 10
+                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, eq, gt, gt, gt, gt, gt, gt, lt, lt, un, un}, // 11
+                {gt, gt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, eq, gt, lt, lt, lt, lt, lt, lt, un, un}, // 12
+                {gt, gt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, eq, lt, lt, lt, lt, lt, lt, un, un}, // 13
+                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, lt, gt, gt, eq, lt, gt, gt, lt, lt, un, un}, // 14
+                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, lt, gt, gt, gt, eq, gt, gt, lt, lt, un, un}, // 15
+                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, lt, gt, gt, lt, lt, eq, gt, lt, lt, un, un}, // 16
+                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, lt, gt, gt, lt, lt, lt, eq, lt, lt, un, un}, // 17
+                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, eq, lt, un, un}, // 18
+                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, eq, un, un}, // 19
+                {un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un}, // 20
+                {un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un}, // 21
+            };
+
+            // check expected partial_ordering against expected booleans
+            REQUIRE(expected.size() == expected_eq.size());
+            REQUIRE(expected.size() == expected_lt.size());
+            for (size_t i = 0; i < expected.size(); ++i)
+            {
+                REQUIRE(expected[i].size() == expected_eq[i].size());
+                REQUIRE(expected[i].size() == expected_lt[i].size());
+                for (size_t j = 0; j < expected[i].size(); ++j)
+                {
+                    CAPTURE(i)
+                    CAPTURE(j)
+                    CHECK(std::is_eq(expected[i][j]) == expected_eq[i][j]);
+                    CHECK(std::is_lt(expected[i][j]) == expected_lt[i][j]);
+                    if (std::is_gt(expected[i][j]))
+                    {
+                        CHECK((!expected_eq[i][j] && !expected_lt[i][j]));
+                    }
+                }
+            }
+
+            // check that two values compare according to their expected ordering
+            REQUIRE(expected.size() == j_values.size());
+            for (size_t i = 0; i < j_values.size(); ++i)
+            {
+                REQUIRE(expected[i].size() == j_values.size());
+                for (size_t j = 0; j < j_values.size(); ++j)
+                {
+                    CAPTURE(i)
+                    CAPTURE(j)
+                    CHECK((j_values[i] <=> j_values[j]) == expected[i][j]); // *NOPAD*
+                }
+            }
+        }
+    }
+
+}
+
+TEST_CASE("regression #3868 - heterogeneous comparisons compile under C++20 (P2468R2)")
+{
+    // Issue #3868: operator!= was preventing compiler from synthesizing reversed
+    // operator== candidates under C++20's P2468R2 rewritten candidate rules.
+    // Verify that heterogeneous comparisons now work.
+
+    SECTION("string vs json")
+    {
+        std::string s = "string";
+        json j = "string";
+        CHECK(s == j);
+        CHECK(j == s);
+        CHECK_FALSE(s != j);
+        CHECK_FALSE(j != s);
+    }
+
+    SECTION("other heterogeneous types")
+    {
+        int i = 42;
+        json j = 42;
+        CHECK(i == j);
+        CHECK(j == i);
+        CHECK_FALSE(i != j);
+        CHECK_FALSE(j != i);
+    }
+}
+
+#if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
+TEST_CASE("regression #5665 - scalar <= discarded and scalar >= discarded in C++20 legacy mode")
+{
+    // Issue #5665: with a scalar on the left-hand side, <= and >= only had the
+    // candidate rewritten from operator<=>, which does not emulate the legacy
+    // discarded-value behavior. Check that scalar-on-the-left now matches the
+    // other three operand orders.
+    const json discarded(json::value_t::discarded);
+    const json one = 1;
+
+    CHECK(discarded <= 1);
+    CHECK(discarded >= 1);
+    CHECK(one <= discarded);
+    CHECK(one >= discarded);
+    CHECK(1 <= discarded);
+    CHECK(1 >= discarded);
+    CHECK(1.5 <= discarded);
+    CHECK(1.5 >= discarded);
+}
+
+#endif
+
+TEST_CASE("containers are compared element by element (C++20)")
+{
+    // Containers nested deeper than a bound are compared without the call
+    // stack, by code of their own; every relation is checked both at the top
+    // level and below that bound.
+    const auto deep = [](const json & j, const std::size_t depth)
+    {
+        json result = j;
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            result = json::array({std::move(result)});
+        }
+        return result;
+    };
+
+    for (const std::size_t depth : std::vector<std::size_t> {0, 200})
+    {
+        CAPTURE(depth)
+
+        // objects with different keys
+        {
+            const json a = deep({{"a", 1}}, depth);
+            const json b = deep({{"b", 1}}, depth);
+            CHECK((a <=> b) == std::partial_ordering::less); // *NOPAD*
+            CHECK((b <=> a) == std::partial_ordering::greater); // *NOPAD*
+            CHECK((a <=> a) == std::partial_ordering::equivalent); // *NOPAD*
+        }
+
+        // a container that is a prefix of the other one
+        {
+            // the one that runs out of elements first is the smaller one
+            const json shorter = deep({1}, depth);
+            const json longer = deep({1, 2}, depth);
+
+            const json smaller_object = deep({{"a", 1}}, depth);
+            const json larger_object = deep({{"a", 1}, {"b", 2}}, depth);
+            CHECK((shorter <=> longer) == std::partial_ordering::less); // *NOPAD*
+            CHECK((longer <=> shorter) == std::partial_ordering::greater); // *NOPAD*
+        }
+
+        // elements that cannot be ordered
+        {
+            const double nan = std::numeric_limits<double>::quiet_NaN();
+            const json lhs = deep({nan, 1}, depth);
+            const json rhs = deep({nan, 2}, depth);
+            // operator<=> stops there, as std::lexicographical_compare_three_way
+            // does, and operator< is derived from it
+            CHECK((lhs <=> rhs) == std::partial_ordering::unordered); // *NOPAD*
+            CHECK_FALSE(lhs < rhs);
+        }
+    }
+}
+
+TEST_CASE("operator<=> of binary values with a different subtype does not depend on nesting depth")
+{
+    // #5654: std::vector<std::uint8_t>::operator<=>, which the binary type's
+    // own operator<=> uses, ignores the subtype that operator== checks. So a
+    // pair of binary values with the same bytes but a different subtype is
+    // unequal, yet <=>-equivalent - the same inconsistency between == and <=>
+    // that a NaN has. Within the nesting bound, an array compares itself
+    // with std::vector's own operator<=>, which treats an equivalent pair as
+    // undecided and lets the next element decide, same as
+    // std::lexicographical_compare_three_way does. Past the bound,
+    // compare_iteratively<true>() takes over and must classify the pair the
+    // same way, or the result of operator<=> - and of <, which C++20 derives
+    // from it - depends on how deeply the values are nested.
+    const json a = json::array({json::binary({1}, 1), 1});
+    const json b = json::array({json::binary({1}, 2), 2});
+
+    // the root inconsistency: unequal, yet <=>-equivalent
+    CHECK_FALSE(a[0] == b[0]);
+    CHECK((a[0] <=> b[0]) == std::partial_ordering::equivalent); // *NOPAD*
+
+    const auto deep = [](const json & j, const std::size_t depth)
+    {
+        json result = j;
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            result = json::array({std::move(result)});
+        }
+        return result;
+    };
+
+    // 127 levels stay within nesting_depth_limit() (128); 128 and 200 do not,
+    // and must still agree with the levels that do
+    for (const std::size_t depth : std::vector<std::size_t> {0, 127, 128, 200})
+    {
+        CAPTURE(depth)
+        const json x = deep(a, depth);
+        const json y = deep(b, depth);
+        CHECK((x <=> y) == std::partial_ordering::less); // *NOPAD*
+        CHECK((y <=> x) == std::partial_ordering::greater); // *NOPAD*
+        CHECK(x < y);
+        CHECK(y > x);
+        CHECK_FALSE(y < x);
+    }
+}
+
+TEST_CASE("comparison of scalars (inline fast path): operator<=>")
+{
+    // the three-way part of "comparison of scalars (inline fast path)" in
+    // unit-comparison.cpp, which is built for C++11 only
+    const json::number_integer_t neg = -1;
+
+    SECTION("NaN")
+    {
+        const json nan = std::numeric_limits<json::number_float_t>::quiet_NaN();
+        const json one = 1.0;
+        CHECK((nan <=> nan) == std::partial_ordering::unordered); // *NOPAD*
+        CHECK((nan <=> one) == std::partial_ordering::unordered); // *NOPAD*
+        CHECK((one <=> nan) == std::partial_ordering::unordered); // *NOPAD*
+        CHECK((one <=> json(2.0)) == std::partial_ordering::less); // *NOPAD*
+        CHECK((json(2.0) <=> one) == std::partial_ordering::greater); // *NOPAD*
+        CHECK((one <=> one) == std::partial_ordering::equivalent); // *NOPAD*
+        CHECK((json(neg) <=> json(1u)) == std::partial_ordering::less); // *NOPAD*
+        CHECK((json(nullptr) <=> json(nullptr)) == std::partial_ordering::equivalent); // *NOPAD*
+        CHECK((json(true) <=> json(false)) == std::partial_ordering::greater); // *NOPAD*
+    }
+
+    SECTION("discarded")
+    {
+        const json d(json::value_t::discarded);
+        CHECK((d <=> d) == std::partial_ordering::unordered); // *NOPAD*
+        CHECK((d <=> json(1)) == std::partial_ordering::unordered); // *NOPAD*
+        CHECK((json(1) <=> d) == std::partial_ordering::unordered); // *NOPAD*
+    }
+}
+
+#endif // JSON_HAS_THREE_WAY_COMPARISON
+#endif

tests/src/unit-comparison.cpp

@@ -6,12 +6,11 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// When using macros that are only defined for particular versions of the standard
-// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
-// version macro in a comment close by, like this:
-// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. To avoid rebuilding this
+// large file for every standard, tests that depend on the standard version (e.g.,
+// those using JSON_HAS_FILESYSTEM, JSON_HAS_RANGES, or JSON_HAS_THREE_WAY_COMPARISON)
+// go into a separate file unit-comparison-cpp<NN>.cpp. This file stays C++11-only.
 
 #include "doctest_compatibility.h"
 
@@ -29,37 +28,6 @@
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
-#if JSON_HAS_THREE_WAY_COMPARISON
-// this can be replaced with the doctest stl extension header in version 2.5
-namespace doctest
-{
-template<> struct StringMaker<std::partial_ordering>
-{
-    static String convert(const std::partial_ordering& order)
-    {
-        if (order == std::partial_ordering::less)
-        {
-            return "std::partial_ordering::less";
-        }
-        if (order == std::partial_ordering::equivalent)
-        {
-            return "std::partial_ordering::equivalent";
-        }
-        if (order == std::partial_ordering::greater)
-        {
-            return "std::partial_ordering::greater";
-        }
-        if (order == std::partial_ordering::unordered)
-        {
-            return "std::partial_ordering::unordered";
-        }
-        return "{?}";
-    }
-};
-} // namespace doctest
-
-#endif
-
 namespace
 {
 // helper function to check std::less<json::value_t>
@@ -76,16 +44,6 @@ TEST_CASE("lexicographical comparison operators")
     constexpr auto f_ = false;
     constexpr auto _t = true;
     constexpr auto nan = std::numeric_limits<json::number_float_t>::quiet_NaN();
-#if JSON_HAS_THREE_WAY_COMPARISON
-    constexpr auto lt = std::partial_ordering::less;
-    constexpr auto gt = std::partial_ordering::greater;
-    constexpr auto eq = std::partial_ordering::equivalent;
-    constexpr auto un = std::partial_ordering::unordered;
-#endif
-
-#if JSON_HAS_THREE_WAY_COMPARISON
-    INFO("using 3-way comparison");
-#endif
 
 #if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
     INFO("using legacy comparison");
@@ -136,62 +94,13 @@ TEST_CASE("lexicographical comparison operators")
                     CAPTURE(i)
                     CAPTURE(j)
                     // check precomputed values
-#if JSON_HAS_THREE_WAY_COMPARISON
-                    // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-                    CHECK((j_types[i] < j_types[j]) == expected_lt[i][j]);
-#else
+#if !JSON_HAS_THREE_WAY_COMPARISON
                     CHECK(operator<(j_types[i], j_types[j]) == expected_lt[i][j]);
 #endif
                     CHECK(f(j_types[i], j_types[j]) == expected_lt[i][j]);
                 }
             }
         }
-#if JSON_HAS_THREE_WAY_COMPARISON
-        // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-        SECTION("comparison: 3-way")
-        {
-            std::vector<std::vector<std::partial_ordering>> expected =
-            {
-                //0   1   2   3   4   5   6   7   8   9
-                {eq, lt, lt, lt, lt, lt, lt, lt, lt, un}, //  0
-                {gt, eq, lt, lt, lt, lt, lt, lt, lt, un}, //  1
-                {gt, gt, eq, eq, eq, lt, lt, lt, lt, un}, //  2
-                {gt, gt, eq, eq, eq, lt, lt, lt, lt, un}, //  3
-                {gt, gt, eq, eq, eq, lt, lt, lt, lt, un}, //  4
-                {gt, gt, gt, gt, gt, eq, lt, lt, lt, un}, //  5
-                {gt, gt, gt, gt, gt, gt, eq, lt, lt, un}, //  6
-                {gt, gt, gt, gt, gt, gt, gt, eq, lt, un}, //  7
-                {gt, gt, gt, gt, gt, gt, gt, gt, eq, un}, //  8
-                {un, un, un, un, un, un, un, un, un, un}, //  9
-            };
-
-            // check expected partial_ordering against expected boolean
-            REQUIRE(expected.size() == expected_lt.size());
-            for (size_t i = 0; i < expected.size(); ++i)
-            {
-                REQUIRE(expected[i].size() == expected_lt[i].size());
-                for (size_t j = 0; j < expected[i].size(); ++j)
-                {
-                    CAPTURE(i)
-                    CAPTURE(j)
-                    CHECK(std::is_lt(expected[i][j]) == expected_lt[i][j]);
-                }
-            }
-
-            // check 3-way comparison against expected partial_ordering
-            REQUIRE(expected.size() == j_types.size());
-            for (size_t i = 0; i < j_types.size(); ++i)
-            {
-                REQUIRE(expected[i].size() == j_types.size());
-                for (size_t j = 0; j < j_types.size(); ++j)
-                {
-                    CAPTURE(i)
-                    CAPTURE(j)
-                    CHECK((j_types[i] <=> j_types[j]) == expected[i][j]); // *NOPAD*
-                }
-            }
-        }
-#endif
     }
 
     SECTION("values")
@@ -320,18 +229,6 @@ TEST_CASE("lexicographical comparison operators")
             CHECK_FALSE(above_int64_max <= max_int64);
             CHECK(above_int64_max > max_int64);
             CHECK(above_int64_max >= max_int64);
-
-#if JSON_HAS_THREE_WAY_COMPARISON
-            // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-            CHECK((negative_one <=> above_int64_max) == std::partial_ordering::less); // *NOPAD*
-            CHECK((above_int64_max <=> negative_one) == std::partial_ordering::greater); // *NOPAD*
-            CHECK((negative_one <=> max_uint64) == std::partial_ordering::less); // *NOPAD*
-            CHECK((max_uint64 <=> negative_one) == std::partial_ordering::greater); // *NOPAD*
-            CHECK((one <=> above_int64_max) == std::partial_ordering::less); // *NOPAD*
-            CHECK((above_int64_max <=> one) == std::partial_ordering::greater); // *NOPAD*
-            CHECK((max_int64 <=> above_int64_max) == std::partial_ordering::less); // *NOPAD*
-            CHECK((above_int64_max <=> max_int64) == std::partial_ordering::greater); // *NOPAD*
-#endif
         }
 
         SECTION("integer/float mixed comparison is exact")
@@ -382,16 +279,6 @@ TEST_CASE("lexicographical comparison operators")
             CHECK_FALSE(json(1) < json(nan));
             CHECK_FALSE(json(nan) < json(1));
             CHECK_FALSE(json(1u) == json(nan));
-
-#if JSON_HAS_THREE_WAY_COMPARISON
-            // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-            CHECK((max_int64 <=> two_63) == std::partial_ordering::less); // *NOPAD*
-            CHECK((two_63 <=> max_int64) == std::partial_ordering::greater); // *NOPAD*
-            CHECK((below_two_63 <=> max_int64) == std::partial_ordering::less); // *NOPAD*
-            CHECK((max_uint64 <=> two_64) == std::partial_ordering::less); // *NOPAD*
-            CHECK((json(1) <=> json(1.0)) == std::partial_ordering::equivalent); // *NOPAD*
-            CHECK((json(1) <=> json(nan)) == std::partial_ordering::unordered); // *NOPAD*
-#endif
         }
 
         SECTION("compares unordered")
@@ -618,72 +505,6 @@ TEST_CASE("lexicographical comparison operators")
                 }
             }
         }
-
-#if JSON_HAS_THREE_WAY_COMPARISON
-        // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-        SECTION("comparison: 3-way")
-        {
-            std::vector<std::vector<std::partial_ordering>> expected =
-            {
-                //0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21
-                {eq, eq, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, un, un}, //  0
-                {eq, eq, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, un, un}, //  1
-                {gt, gt, eq, lt, lt, lt, lt, lt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  2
-                {gt, gt, gt, eq, gt, gt, gt, gt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  3
-                {gt, gt, gt, lt, eq, lt, gt, lt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  4
-                {gt, gt, gt, lt, gt, eq, gt, lt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  5
-                {gt, gt, gt, lt, lt, lt, eq, lt, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  6
-                {gt, gt, gt, lt, gt, gt, gt, eq, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  7
-                {gt, gt, un, un, un, un, un, un, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  8
-                {gt, gt, un, un, un, un, un, un, un, un, lt, lt, gt, gt, lt, lt, lt, lt, lt, lt, un, un}, //  9
-                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, eq, gt, gt, gt, gt, gt, gt, gt, lt, lt, un, un}, // 10
-                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, eq, gt, gt, gt, gt, gt, gt, lt, lt, un, un}, // 11
-                {gt, gt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, eq, gt, lt, lt, lt, lt, lt, lt, un, un}, // 12
-                {gt, gt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, lt, eq, lt, lt, lt, lt, lt, lt, un, un}, // 13
-                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, lt, gt, gt, eq, lt, gt, gt, lt, lt, un, un}, // 14
-                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, lt, gt, gt, gt, eq, gt, gt, lt, lt, un, un}, // 15
-                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, lt, gt, gt, lt, lt, eq, gt, lt, lt, un, un}, // 16
-                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, lt, lt, gt, gt, lt, lt, lt, eq, lt, lt, un, un}, // 17
-                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, eq, lt, un, un}, // 18
-                {gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, gt, eq, un, un}, // 19
-                {un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un}, // 20
-                {un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un, un}, // 21
-            };
-
-            // check expected partial_ordering against expected booleans
-            REQUIRE(expected.size() == expected_eq.size());
-            REQUIRE(expected.size() == expected_lt.size());
-            for (size_t i = 0; i < expected.size(); ++i)
-            {
-                REQUIRE(expected[i].size() == expected_eq[i].size());
-                REQUIRE(expected[i].size() == expected_lt[i].size());
-                for (size_t j = 0; j < expected[i].size(); ++j)
-                {
-                    CAPTURE(i)
-                    CAPTURE(j)
-                    CHECK(std::is_eq(expected[i][j]) == expected_eq[i][j]);
-                    CHECK(std::is_lt(expected[i][j]) == expected_lt[i][j]);
-                    if (std::is_gt(expected[i][j]))
-                    {
-                        CHECK((!expected_eq[i][j] && !expected_lt[i][j]));
-                    }
-                }
-            }
-
-            // check that two values compare according to their expected ordering
-            REQUIRE(expected.size() == j_values.size());
-            for (size_t i = 0; i < j_values.size(); ++i)
-            {
-                REQUIRE(expected[i].size() == j_values.size());
-                for (size_t j = 0; j < j_values.size(); ++j)
-                {
-                    CAPTURE(i)
-                    CAPTURE(j)
-                    CHECK((j_values[i] <=> j_values[j]) == expected[i][j]); // *NOPAD*
-                }
-            }
-        }
-#endif
     }
 
 #if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
@@ -722,60 +543,7 @@ TEST_CASE("lexicographical comparison operators")
 #endif
 }
 
-#if JSON_HAS_THREE_WAY_COMPARISON
-// JSON_HAS_CPP_20 (do not remove; see note at top of file)
-
-TEST_CASE("regression #3868 - heterogeneous comparisons compile under C++20 (P2468R2)")
-{
-    // Issue #3868: operator!= was preventing compiler from synthesizing reversed
-    // operator== candidates under C++20's P2468R2 rewritten candidate rules.
-    // Verify that heterogeneous comparisons now work.
-
-    SECTION("string vs json")
-    {
-        std::string s = "string";
-        json j = "string";
-        CHECK(s == j);
-        CHECK(j == s);
-        CHECK_FALSE(s != j);
-        CHECK_FALSE(j != s);
-    }
-
-    SECTION("other heterogeneous types")
-    {
-        int i = 42;
-        json j = 42;
-        CHECK(i == j);
-        CHECK(j == i);
-        CHECK_FALSE(i != j);
-        CHECK_FALSE(j != i);
-    }
-}
-
-#if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
-TEST_CASE("regression #5665 - scalar <= discarded and scalar >= discarded in C++20 legacy mode")
-{
-    // Issue #5665: with a scalar on the left-hand side, <= and >= only had the
-    // candidate rewritten from operator<=>, which does not emulate the legacy
-    // discarded-value behavior. Check that scalar-on-the-left now matches the
-    // other three operand orders.
-    const json discarded(json::value_t::discarded);
-    const json one = 1;
-
-    CHECK(discarded <= 1);
-    CHECK(discarded >= 1);
-    CHECK(one <= discarded);
-    CHECK(one >= discarded);
-    CHECK(1 <= discarded);
-    CHECK(1 >= discarded);
-    CHECK(1.5 <= discarded);
-    CHECK(1.5 >= discarded);
-}
-#endif
-
-#endif
-
-namespace
+namespace unit_comparison_detail
 {
 // orders keys ascending or descending, as chosen when a map is created
 template<class Key>
@@ -827,29 +595,6 @@ struct unordered_object_t : std::map<Key, Value, directed_less<Key>, Allocator>
 };
 using unordered_json = nlohmann::json::with_object_t<unordered_object_t>;
 
-// the entries "0" to "9", enumerated in ascending or in descending order
-unordered_json make_unordered_object(const bool descending)
-{
-    unordered_json j = unordered_json::object_t(directed_less<std::string>(descending));
-    for (int i = 0; i < 10; ++i)
-    {
-        j[std::to_string(i)] = i;
-    }
-    return j;
-}
-
-template<typename Json>
-Json nest(Json j, const std::size_t depth)
-{
-    for (std::size_t i = 0; i < depth; ++i)
-    {
-        Json outer = Json::object();
-        outer["x"] = std::move(j);
-        j = std::move(outer);
-    }
-    return j;
-}
-
 // a std::map comparator with state: case-insensitive, unless constructed
 // case-sensitive. Used to check that copying an object copies the original's
 // comparator rather than default-constructing a new one (see #5649).
@@ -878,18 +623,6 @@ template<class Key, class Value, class /*Compare*/, class Allocator>
 using key_case_map = std::map<Key, Value, key_case_less, Allocator>;
 using key_case_json = nlohmann::json::with_object_t<key_case_map>;
 
-// the innermost value of a chain of single-element arrays
-template<typename Json>
-const Json& innermost(const Json& j)
-{
-    const Json* p = &j;
-    while (p->is_array())
-    {
-        p = &(*p)[0];
-    }
-    return *p;
-}
-
 // orders keys case-insensitively, so "key" and "KEY" compare equivalent
 // (neither less than the other) although they are not equal
 struct case_insensitive_less
@@ -907,7 +640,58 @@ struct case_insensitive_less
 template<class Key, class Value, class /*Compare*/, class Allocator>
 using case_insensitive_map = std::map<Key, Value, case_insensitive_less, Allocator>;
 using ci_json = nlohmann::json::with_object_t<case_insensitive_map>;
+
+// the types above keep external linkage: a basic_json specialized with a type
+// of an anonymous namespace has internal linkage, which GCC reports in a file
+// #include-d into a batch (-Wsubobject-linkage); the functions need none
+namespace
+{
+// the entries "0" to "9", enumerated in ascending or in descending order
+unordered_json make_unordered_object(const bool descending)
+{
+    unordered_json j = unordered_json::object_t(directed_less<std::string>(descending));
+    for (int i = 0; i < 10; ++i)
+    {
+        j[std::to_string(i)] = i;
+    }
+    return j;
+}
+
+template<typename Json>
+Json nest(Json j, const std::size_t depth)
+{
+    for (std::size_t i = 0; i < depth; ++i)
+    {
+        Json outer = Json::object();
+        outer["x"] = std::move(j);
+        j = std::move(outer);
+    }
+    return j;
+}
+
+// the innermost value of a chain of single-element arrays
+template<typename Json>
+const Json& innermost(const Json& j)
+{
+    const Json* p = &j;
+    while (p->is_array())
+    {
+        p = &(*p)[0];
+    }
+    return *p;
+}
 } // namespace
+} // namespace unit_comparison_detail
+
+// using-declarations rather than a using-directive, as this file may be
+// #include-d into a batch with other test files (JSON_TestUnityBuild)
+using unit_comparison_detail::ci_json;
+using unit_comparison_detail::innermost;
+using unit_comparison_detail::key_case_json;
+using unit_comparison_detail::key_case_less;
+using unit_comparison_detail::make_unordered_object;
+using unit_comparison_detail::nest;
+using unit_comparison_detail::unordered_json;
 
 TEST_CASE("equality of objects whose entries have no fixed order")
 {
@@ -1050,12 +834,6 @@ TEST_CASE("containers are compared element by element")
             CHECK(a < b);
             CHECK(b > a);
             CHECK_FALSE(b < a);
-#if JSON_HAS_THREE_WAY_COMPARISON
-            // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-            CHECK((a <=> b) == std::partial_ordering::less); // *NOPAD*
-            CHECK((b <=> a) == std::partial_ordering::greater); // *NOPAD*
-            CHECK((a <=> a) == std::partial_ordering::equivalent); // *NOPAD*
-#endif
         }
 
         // a container that is a prefix of the other one
@@ -1073,11 +851,6 @@ TEST_CASE("containers are compared element by element")
             CHECK(smaller_object < larger_object);
             CHECK(larger_object > smaller_object);
             CHECK_FALSE(smaller_object == larger_object);
-#if JSON_HAS_THREE_WAY_COMPARISON
-            // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-            CHECK((shorter <=> longer) == std::partial_ordering::less); // *NOPAD*
-            CHECK((longer <=> shorter) == std::partial_ordering::greater); // *NOPAD*
-#endif
         }
 
         // elements that cannot be ordered
@@ -1088,13 +861,7 @@ TEST_CASE("containers are compared element by element")
 
             CHECK_FALSE(lhs == lhs);
             CHECK_FALSE(rhs < lhs);
-#if JSON_HAS_THREE_WAY_COMPARISON
-            // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-            // operator<=> stops there, as std::lexicographical_compare_three_way
-            // does, and operator< is derived from it
-            CHECK((lhs <=> rhs) == std::partial_ordering::unordered); // *NOPAD*
-            CHECK_FALSE(lhs < rhs);
-#else
+#if !JSON_HAS_THREE_WAY_COMPARISON
             // operator< skips a pair of elements that cannot be ordered, as
             // std::lexicographical_compare does, and the next pair decides
             CHECK(lhs < rhs);
@@ -1103,54 +870,6 @@ TEST_CASE("containers are compared element by element")
     }
 }
 
-#if JSON_HAS_THREE_WAY_COMPARISON
-// JSON_HAS_CPP_20 (do not remove; see note at top of file)
-TEST_CASE("operator<=> of binary values with a different subtype does not depend on nesting depth")
-{
-    // #5654: std::vector<std::uint8_t>::operator<=>, which the binary type's
-    // own operator<=> uses, ignores the subtype that operator== checks. So a
-    // pair of binary values with the same bytes but a different subtype is
-    // unequal, yet <=>-equivalent - the same inconsistency between == and <=>
-    // that a NaN has. Within the nesting bound, an array compares itself
-    // with std::vector's own operator<=>, which treats an equivalent pair as
-    // undecided and lets the next element decide, same as
-    // std::lexicographical_compare_three_way does. Past the bound,
-    // compare_iteratively<true>() takes over and must classify the pair the
-    // same way, or the result of operator<=> - and of <, which C++20 derives
-    // from it - depends on how deeply the values are nested.
-    const json a = json::array({json::binary({1}, 1), 1});
-    const json b = json::array({json::binary({1}, 2), 2});
-
-    // the root inconsistency: unequal, yet <=>-equivalent
-    CHECK_FALSE(a[0] == b[0]);
-    CHECK((a[0] <=> b[0]) == std::partial_ordering::equivalent); // *NOPAD*
-
-    const auto deep = [](const json & j, const std::size_t depth)
-    {
-        json result = j;
-        for (std::size_t i = 0; i < depth; ++i)
-        {
-            result = json::array({std::move(result)});
-        }
-        return result;
-    };
-
-    // 127 levels stay within nesting_depth_limit() (128); 128 and 200 do not,
-    // and must still agree with the levels that do
-    for (const std::size_t depth : std::vector<std::size_t> {0, 127, 128, 200})
-    {
-        CAPTURE(depth)
-        const json x = deep(a, depth);
-        const json y = deep(b, depth);
-        CHECK((x <=> y) == std::partial_ordering::less); // *NOPAD*
-        CHECK((y <=> x) == std::partial_ordering::greater); // *NOPAD*
-        CHECK(x < y);
-        CHECK(y > x);
-        CHECK_FALSE(y < x);
-    }
-}
-#endif
-
 TEST_CASE("comparison of scalars (inline fast path)")
 {
     const json::number_integer_t neg = -1;
@@ -1249,17 +968,6 @@ TEST_CASE("comparison of scalars (inline fast path)")
         CHECK_FALSE(nan < json(1u));
         CHECK_FALSE(json(1u) < nan);
 
-#if JSON_HAS_THREE_WAY_COMPARISON
-        CHECK((nan <=> nan) == std::partial_ordering::unordered); // *NOPAD*
-        CHECK((nan <=> one) == std::partial_ordering::unordered); // *NOPAD*
-        CHECK((one <=> nan) == std::partial_ordering::unordered); // *NOPAD*
-        CHECK((one <=> json(2.0)) == std::partial_ordering::less); // *NOPAD*
-        CHECK((json(2.0) <=> one) == std::partial_ordering::greater); // *NOPAD*
-        CHECK((one <=> one) == std::partial_ordering::equivalent); // *NOPAD*
-        CHECK((json(neg) <=> json(1u)) == std::partial_ordering::less); // *NOPAD*
-        CHECK((json(nullptr) <=> json(nullptr)) == std::partial_ordering::equivalent); // *NOPAD*
-        CHECK((json(true) <=> json(false)) == std::partial_ordering::greater); // *NOPAD*
-#endif
     }
 
     SECTION("discarded")
@@ -1275,11 +983,6 @@ TEST_CASE("comparison of scalars (inline fast path)")
         CHECK_FALSE(d == json(nullptr));
         CHECK_FALSE(json(nullptr) == d);
 
-#if JSON_HAS_THREE_WAY_COMPARISON
-        CHECK((d <=> d) == std::partial_ordering::unordered); // *NOPAD*
-        CHECK((d <=> json(1)) == std::partial_ordering::unordered); // *NOPAD*
-        CHECK((json(1) <=> d) == std::partial_ordering::unordered); // *NOPAD*
-#endif
     }
 
     SECTION("scalar overloads")

tests/src/unit-conversions1-cpp17.cpp

@@ -0,0 +1,95 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-conversions1.cpp (conversions to and from
+// std::string_view). It is kept in a separate translation unit so the (much larger) unit-
+// conversions1.cpp is built for C++11 only and not rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+// workaround for MSVC, which does not set __cplusplus to the language version (#464)
+#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
+    #define JSON_HAS_CPP_17
+#endif
+
+#ifdef JSON_HAS_CPP_17
+#include <string>
+#include <string_view>
+
+#if __has_include(<optional>)
+    #include <optional>
+#elif __has_include(<experimental/optional>)
+    #include <experimental/optional>
+#endif
+
+TEST_CASE("value conversion (C++17)")
+{
+    SECTION("get a string (explicit)")
+    {
+        const json::string_t s_reference{"Hello world"};
+        json j(s_reference);
+
+        SECTION("std::string_view")
+        {
+            std::string_view const s = j.get<std::string_view>();
+            CHECK(json(s) == j);
+        }
+
+        SECTION("exception in case of a non-string type using string_view")
+        {
+            CHECK_THROWS_WITH_AS(json(json::value_t::null).get<std::string_view>(),
+                                 "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
+            CHECK_THROWS_WITH_AS(json(json::value_t::object).get<std::string_view>(),
+                                 "[json.exception.type_error.302] type must be string, but is object", json::type_error&);
+            CHECK_THROWS_WITH_AS(json(json::value_t::array).get<std::string_view>(),
+                                 "[json.exception.type_error.302] type must be string, but is array", json::type_error&);
+            CHECK_THROWS_WITH_AS(json(json::value_t::boolean).get<std::string_view>(),
+                                 "[json.exception.type_error.302] type must be string, but is boolean", json::type_error&);
+            CHECK_THROWS_WITH_AS(json(json::value_t::number_integer).get<std::string_view>(),
+                                 "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
+            CHECK_THROWS_WITH_AS(json(json::value_t::number_unsigned).get<std::string_view>(),
+                                 "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
+            CHECK_THROWS_WITH_AS(json(json::value_t::number_float).get<std::string_view>(),
+                                 "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
+        }
+    }
+
+    SECTION("get a string (explicit, get_to)")
+    {
+        const json::string_t s_reference{"Hello world"};
+        json j(s_reference);
+
+        SECTION("std::string_view")
+        {
+            std::string const s = "previous value";
+            std::string_view sv = s;
+            j.get_to(sv);
+            CHECK(json(sv) == j);
+        }
+    }
+
+#if JSON_USE_IMPLICIT_CONVERSIONS
+    SECTION("get a string (implicit)")
+    {
+        const json::string_t s_reference{"Hello world"};
+        json j(s_reference);
+
+        SECTION("std::string_view")
+        {
+            std::string_view const s = j.get<std::string_view>();
+            CHECK(json(s) == j);
+        }
+    }
+#endif
+
+}
+
+#endif

tests/src/unit-conversions1.cpp

@@ -6,12 +6,11 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// When using macros that are only defined for particular versions of the standard
-// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
-// version macro in a comment close by, like this:
-// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. To avoid rebuilding this
+// large file for every standard, tests that depend on the standard version (e.g.,
+// those using JSON_HAS_FILESYSTEM, JSON_HAS_RANGES, or JSON_HAS_THREE_WAY_COMPARISON)
+// go into a separate file unit-conversions1-cpp<NN>.cpp. This file stays C++11-only.
 
 #include "doctest_compatibility.h"
 
@@ -31,25 +30,6 @@ using nlohmann::json;
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
 DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
 
-#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
-    #define JSON_HAS_CPP_17
-    #define JSON_HAS_CPP_14
-#elif (defined(__cplusplus) && __cplusplus >= 201402L) || (defined(_HAS_CXX14) && _HAS_CXX14 == 1)
-    #define JSON_HAS_CPP_14
-#endif
-
-#ifdef JSON_HAS_CPP_17
-    #if __has_include(<optional>)
-        #include <optional>
-    #elif __has_include(<experimental/optional>)
-        #include <experimental/optional>
-    #endif
-#endif
-
-#if defined(JSON_HAS_CPP_17)
-    #include <string_view>
-#endif
-
 TEST_CASE("value conversion")
 {
     SECTION("get an object (explicit)")
@@ -519,13 +499,6 @@ TEST_CASE("value conversion")
             const std::string s = j.get<std::string>();
             CHECK(json(s) == j);
         }
-#if defined(JSON_HAS_CPP_17)
-        SECTION("std::string_view")
-        {
-            std::string_view const s = j.get<std::string_view>();
-            CHECK(json(s) == j);
-        }
-#endif
 
         SECTION("exception in case of a non-string type")
         {
@@ -551,26 +524,6 @@ TEST_CASE("value conversion")
                 json(json::value_t::number_float).get<json::string_t>(),
                 "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
         }
-
-#if defined(JSON_HAS_CPP_17)
-        SECTION("exception in case of a non-string type using string_view")
-        {
-            CHECK_THROWS_WITH_AS(json(json::value_t::null).get<std::string_view>(),
-                                 "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
-            CHECK_THROWS_WITH_AS(json(json::value_t::object).get<std::string_view>(),
-                                 "[json.exception.type_error.302] type must be string, but is object", json::type_error&);
-            CHECK_THROWS_WITH_AS(json(json::value_t::array).get<std::string_view>(),
-                                 "[json.exception.type_error.302] type must be string, but is array", json::type_error&);
-            CHECK_THROWS_WITH_AS(json(json::value_t::boolean).get<std::string_view>(),
-                                 "[json.exception.type_error.302] type must be string, but is boolean", json::type_error&);
-            CHECK_THROWS_WITH_AS(json(json::value_t::number_integer).get<std::string_view>(),
-                                 "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
-            CHECK_THROWS_WITH_AS(json(json::value_t::number_unsigned).get<std::string_view>(),
-                                 "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
-            CHECK_THROWS_WITH_AS(json(json::value_t::number_float).get<std::string_view>(),
-                                 "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
-        }
-#endif
     }
 
     SECTION("get a string (explicit, get_to)")
@@ -591,15 +544,6 @@ TEST_CASE("value conversion")
             j.get_to(s);
             CHECK(json(s) == j);
         }
-#if defined(JSON_HAS_CPP_17)
-        SECTION("std::string_view")
-        {
-            std::string const s = "previous value";
-            std::string_view sv = s;
-            j.get_to(sv);
-            CHECK(json(sv) == j);
-        }
-#endif
     }
 
     SECTION("get null (explicit)")
@@ -638,14 +582,6 @@ TEST_CASE("value conversion")
             CHECK(json(s) == j);
         }
 
-#if defined(JSON_HAS_CPP_17)
-        SECTION("std::string_view")
-        {
-            std::string_view const s = j.get<std::string_view>();
-            CHECK(json(s) == j);
-        }
-#endif
-
         SECTION("std::string")
         {
             const std::string s = j;
@@ -1256,11 +1192,4 @@ TEST_CASE("value conversion")
 #endif
 }
 
-#ifdef JSON_HAS_CPP_17
-    #undef JSON_HAS_CPP_17
-#endif
-
-#ifdef JSON_HAS_CPP_14
-    #undef JSON_HAS_CPP_14
-#endif
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-conversions2-cpp17.cpp

@@ -0,0 +1,225 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-conversions2.cpp (conversions of
+// std::filesystem::path, std::u8string and std::optional). It is kept in a separate
+// translation unit so the (much larger) unit-conversions2.cpp is built for C++11 only and
+// not rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+// workaround for MSVC, which does not set __cplusplus to the language version (#464)
+#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
+    #define JSON_HAS_CPP_17
+#endif
+
+// NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair
+DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
+DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
+
+#ifdef JSON_HAS_CPP_17
+#include <map>
+#include <stdexcept>
+#include <string>
+#include <string_view>
+#include <type_traits>
+#include <vector>
+
+#if __has_include(<optional>)
+    #include <optional>
+#elif __has_include(<experimental/optional>)
+    #include <experimental/optional>
+#endif
+
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+TEST_CASE("std::filesystem::path")
+{
+    SECTION("ascii")
+    {
+        json const j_string = "Path";
+        auto p = j_string.template get<nlohmann::detail::std_fs::path>();
+        json const j_path = p;
+
+        CHECK(j_path.template get<std::string>() ==
+              j_string.template get<std::string>());
+    }
+
+    SECTION("utf-8")
+    {
+        json const j_string = "P\xc4\x9b\xc5\xa1ina";
+        auto p = j_string.template get<nlohmann::detail::std_fs::path>();
+        json const j_path = p;
+
+        CHECK(j_path.template get<std::string>() ==
+              j_string.template get<std::string>());
+    }
+}
+
+#endif
+
+// the ADL to_json overload for std::u8string only exists under the same guard
+// as std::filesystem::path support (it is otherwise only reached indirectly,
+// via std::filesystem::path::u8string()) -- mirror both #if conditions from
+// include/nlohmann/detail/conversions/to_json.hpp exactly
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+#if defined(__cpp_lib_char8_t)
+TEST_CASE("std::u8string")
+{
+    SECTION("ascii")
+    {
+        const std::u8string s = u8"Path";
+        json const j = s;
+
+        CHECK(j.template get<std::string>() == "Path");
+    }
+
+    SECTION("utf-8")
+    {
+        // use \u universal-character-names (rather than raw \x byte escapes
+        // or literal non-ASCII source bytes) to compose the multi-byte UTF-8
+        // encoding -- MSVC treats \x escapes used that way inside a u8
+        // literal as a nonstandard extension (warning C5321), which some of
+        // our CI configs promote to an error; \u is portable and produces
+        // the exact same encoded bytes without depending on the source
+        // file's encoding
+        const std::u8string s = u8"P\u011B\u0161ina";
+        json const j = s;
+
+        CHECK(j.template get<std::string>() == "P\xc4\x9b\xc5\xa1ina");
+    }
+}
+
+#endif
+#endif
+
+#if !defined(JSON_NOEXCEPTION)
+namespace
+{
+// a type whose to_json reports an error by throwing, used below to check that
+// converting a std::optional<T> to JSON propagates an exception thrown while
+// converting its contained value instead of calling std::terminate (#5642)
+struct throwing_to_json_type {};
+
+[[noreturn]] void to_json(json& /*unused*/, const throwing_to_json_type& /*unused*/)
+{
+    throw std::runtime_error("cannot serialize throwing_to_json_type");
+}
+}  // namespace
+#endif
+
+TEST_CASE("std::optional")
+{
+    SECTION("null")
+    {
+        const json j_null;
+        const std::optional<std::string> opt_null;
+
+        CHECK(json(opt_null) == j_null);
+        CHECK(j_null.get<std::optional<std::string>>() == std::nullopt);
+
+        // Constructing std::optional<T> directly from JSON null throws because
+        // std::optional's own converting constructor is chosen over basic_json's
+        // operator T(). This is a language-level limitation (std::optional<T> is
+        // constructible from T, and T is constructible from basic_json via the
+        // operator); there is no SFINAE path that distinguishes "call from inside
+        // std::optional's constructor" from "direct call". Use get<std::optional<T>>()
+        // or get_to() instead for correct null handling. See #4864 and #5246.
+        CHECK_THROWS_WITH_AS(std::optional<std::string>(j_null),
+                             "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
+        CHECK_THROWS_WITH_AS(std::optional<int>(j_null),
+                             "[json.exception.type_error.302] type must be number, but is null", json::type_error&);
+
+        // Assignment goes through the same overload resolution as direct
+        // construction, so it throws for the same reason. This relies on
+        // basic_json's implicit conversion operator, so it only applies
+        // when JSON_USE_IMPLICIT_CONVERSIONS is enabled (the default).
+#if JSON_USE_IMPLICIT_CONVERSIONS
+        std::optional<std::string> opt_assign;
+        CHECK_THROWS_WITH_AS(opt_assign = j_null,
+                             "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
+#endif
+
+        // get_to() is the correct way to obtain std::nullopt from a JSON null.
+        std::optional<std::string> opt_get_to = "placeholder";
+        j_null.get_to(opt_get_to);
+        CHECK(opt_get_to == std::nullopt);
+    }
+
+    SECTION("string")
+    {
+        json j_string = "string";
+        std::optional<std::string> opt_string = "string";
+
+        CHECK(json(opt_string) == j_string);
+        CHECK(std::optional<std::string>(j_string) == opt_string);
+        // false positive: Infer attributes the destruction of the temporaries above to opt_string
+        // @infer-ignore USE_AFTER_DELETE
+    }
+
+    SECTION("bool")
+    {
+        json j_bool = true;
+        std::optional<bool> opt_bool = true;
+
+        CHECK(json(opt_bool) == j_bool);
+        CHECK(std::optional<bool>(j_bool) == opt_bool);
+    }
+
+    SECTION("number")
+    {
+        json j_number = 1;
+        std::optional<int> opt_int = 1;
+
+        CHECK(json(opt_int) == j_number);
+        CHECK(j_number.get<std::optional<int>>() == opt_int);
+    }
+
+    SECTION("array")
+    {
+        json j_array = {1, 2, nullptr};
+        std::vector<std::optional<int>> opt_array = {{1, 2, std::nullopt}};
+
+        CHECK(json(opt_array) == j_array);
+        CHECK(j_array.get<std::vector<std::optional<int>>>() == opt_array);
+    }
+
+    SECTION("object")
+    {
+        json j_object = {{"one", 1}, {"two", 2}, {"zero", nullptr}};
+        std::map<std::string, std::optional<int>> opt_object {{"one", 1}, {"two", 2}, {"zero", std::nullopt}};
+
+        CHECK(json(opt_object) == j_object);
+        CHECK(std::map<std::string, std::optional<int>>(j_object) == opt_object);
+    }
+
+#if !defined(JSON_NOEXCEPTION)
+    SECTION("exception from contained value's to_json propagates (#5642)")
+    {
+        // to_json(BasicJsonType&, const std::optional<T>&) must not be
+        // noexcept: it calls T's to_json, which may throw (a user-defined
+        // to_json that reports an error, or std::bad_alloc for T =
+        // std::string/vector/json). Before the fix, this called
+        // std::terminate() instead of letting the exception propagate.
+        const std::optional<throwing_to_json_type> opt = throwing_to_json_type{};
+        CHECK_THROWS_WITH_AS(json(opt), "cannot serialize throwing_to_json_type", std::runtime_error&);
+
+        // the conversion is noexcept exactly when converting the contained value is
+        // (except with MSVC 2017, where it is never noexcept, see to_json.hpp)
+#if !defined(_MSC_VER) || defined(__clang__) || _MSC_VER >= 1920
+        static_assert(!std::is_nothrow_constructible<json, const std::optional<throwing_to_json_type>&>::value);
+        static_assert(std::is_nothrow_constructible<json, const std::optional<int>&>::value);
+#endif
+    }
+#endif
+}
+
+DOCTEST_CLANG_SUPPRESS_WARNING_POP
+#endif

tests/src/unit-conversions2.cpp

@@ -6,12 +6,11 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// When using macros that are only defined for particular versions of the standard
-// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
-// version macro in a comment close by, like this:
-// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. To avoid rebuilding this
+// large file for every standard, tests that depend on the standard version (e.g.,
+// those using JSON_HAS_FILESYSTEM, JSON_HAS_RANGES, or JSON_HAS_THREE_WAY_COMPARISON)
+// go into a separate file unit-conversions2-cpp<NN>.cpp. This file stays C++11-only.
 
 #include "doctest_compatibility.h"
 
@@ -36,25 +35,6 @@ using nlohmann::json;
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
 DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
 
-#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
-    #define JSON_HAS_CPP_17
-    #define JSON_HAS_CPP_14
-#elif (defined(__cplusplus) && __cplusplus >= 201402L) || (defined(_HAS_CXX14) && _HAS_CXX14 == 1)
-    #define JSON_HAS_CPP_14
-#endif
-
-#ifdef JSON_HAS_CPP_17
-    #if __has_include(<optional>)
-        #include <optional>
-    #elif __has_include(<experimental/optional>)
-        #include <experimental/optional>
-    #endif
-#endif
-
-#if defined(JSON_HAS_CPP_17)
-    #include <string_view>
-#endif
-
 TEST_CASE("value conversion")
 {
     SECTION("get a binary value (explicit)")
@@ -680,194 +660,4 @@ TEST_CASE("Strict JSON to enum mapping")
     }
 }
 
-
-#ifdef JSON_HAS_CPP_17
-#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
-TEST_CASE("std::filesystem::path")
-{
-    SECTION("ascii")
-    {
-        json const j_string = "Path";
-        auto p = j_string.template get<nlohmann::detail::std_fs::path>();
-        json const j_path = p;
-
-        CHECK(j_path.template get<std::string>() ==
-              j_string.template get<std::string>());
-    }
-
-    SECTION("utf-8")
-    {
-        json const j_string = "P\xc4\x9b\xc5\xa1ina";
-        auto p = j_string.template get<nlohmann::detail::std_fs::path>();
-        json const j_path = p;
-
-        CHECK(j_path.template get<std::string>() ==
-              j_string.template get<std::string>());
-    }
-}
-#endif
-
-// the ADL to_json overload for std::u8string only exists under the same guard
-// as std::filesystem::path support (it is otherwise only reached indirectly,
-// via std::filesystem::path::u8string()) -- mirror both #if conditions from
-// include/nlohmann/detail/conversions/to_json.hpp exactly
-#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
-#if defined(__cpp_lib_char8_t)
-TEST_CASE("std::u8string")
-{
-    SECTION("ascii")
-    {
-        const std::u8string s = u8"Path";
-        json const j = s;
-
-        CHECK(j.template get<std::string>() == "Path");
-    }
-
-    SECTION("utf-8")
-    {
-        // use \u universal-character-names (rather than raw \x byte escapes
-        // or literal non-ASCII source bytes) to compose the multi-byte UTF-8
-        // encoding -- MSVC treats \x escapes used that way inside a u8
-        // literal as a nonstandard extension (warning C5321), which some of
-        // our CI configs promote to an error; \u is portable and produces
-        // the exact same encoded bytes without depending on the source
-        // file's encoding
-        const std::u8string s = u8"P\u011B\u0161ina";
-        json const j = s;
-
-        CHECK(j.template get<std::string>() == "P\xc4\x9b\xc5\xa1ina");
-    }
-}
-#endif
-#endif
-
-#if !defined(JSON_NOEXCEPTION)
-namespace
-{
-// a type whose to_json reports an error by throwing, used below to check that
-// converting a std::optional<T> to JSON propagates an exception thrown while
-// converting its contained value instead of calling std::terminate (#5642)
-struct throwing_to_json_type {};
-
-[[noreturn]] void to_json(json& /*unused*/, const throwing_to_json_type& /*unused*/)
-{
-    throw std::runtime_error("cannot serialize throwing_to_json_type");
-}
-}  // namespace
-#endif
-
-TEST_CASE("std::optional")
-{
-    SECTION("null")
-    {
-        const json j_null;
-        const std::optional<std::string> opt_null;
-
-        CHECK(json(opt_null) == j_null);
-        CHECK(j_null.get<std::optional<std::string>>() == std::nullopt);
-
-        // Constructing std::optional<T> directly from JSON null throws because
-        // std::optional's own converting constructor is chosen over basic_json's
-        // operator T(). This is a language-level limitation (std::optional<T> is
-        // constructible from T, and T is constructible from basic_json via the
-        // operator); there is no SFINAE path that distinguishes "call from inside
-        // std::optional's constructor" from "direct call". Use get<std::optional<T>>()
-        // or get_to() instead for correct null handling. See #4864 and #5246.
-        CHECK_THROWS_WITH_AS(std::optional<std::string>(j_null),
-                             "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
-        CHECK_THROWS_WITH_AS(std::optional<int>(j_null),
-                             "[json.exception.type_error.302] type must be number, but is null", json::type_error&);
-
-        // Assignment goes through the same overload resolution as direct
-        // construction, so it throws for the same reason. This relies on
-        // basic_json's implicit conversion operator, so it only applies
-        // when JSON_USE_IMPLICIT_CONVERSIONS is enabled (the default).
-#if JSON_USE_IMPLICIT_CONVERSIONS
-        std::optional<std::string> opt_assign;
-        CHECK_THROWS_WITH_AS(opt_assign = j_null,
-                             "[json.exception.type_error.302] type must be string, but is null", json::type_error&);
-#endif
-
-        // get_to() is the correct way to obtain std::nullopt from a JSON null.
-        std::optional<std::string> opt_get_to = "placeholder";
-        j_null.get_to(opt_get_to);
-        CHECK(opt_get_to == std::nullopt);
-    }
-
-    SECTION("string")
-    {
-        json j_string = "string";
-        std::optional<std::string> opt_string = "string";
-
-        CHECK(json(opt_string) == j_string);
-        CHECK(std::optional<std::string>(j_string) == opt_string);
-        // false positive: Infer attributes the destruction of the temporaries above to opt_string
-        // @infer-ignore USE_AFTER_DELETE
-    }
-
-    SECTION("bool")
-    {
-        json j_bool = true;
-        std::optional<bool> opt_bool = true;
-
-        CHECK(json(opt_bool) == j_bool);
-        CHECK(std::optional<bool>(j_bool) == opt_bool);
-    }
-
-    SECTION("number")
-    {
-        json j_number = 1;
-        std::optional<int> opt_int = 1;
-
-        CHECK(json(opt_int) == j_number);
-        CHECK(j_number.get<std::optional<int>>() == opt_int);
-    }
-
-    SECTION("array")
-    {
-        json j_array = {1, 2, nullptr};
-        std::vector<std::optional<int>> opt_array = {{1, 2, std::nullopt}};
-
-        CHECK(json(opt_array) == j_array);
-        CHECK(j_array.get<std::vector<std::optional<int>>>() == opt_array);
-    }
-
-    SECTION("object")
-    {
-        json j_object = {{"one", 1}, {"two", 2}, {"zero", nullptr}};
-        std::map<std::string, std::optional<int>> opt_object {{"one", 1}, {"two", 2}, {"zero", std::nullopt}};
-
-        CHECK(json(opt_object) == j_object);
-        CHECK(std::map<std::string, std::optional<int>>(j_object) == opt_object);
-    }
-
-#if !defined(JSON_NOEXCEPTION)
-    SECTION("exception from contained value's to_json propagates (#5642)")
-    {
-        // to_json(BasicJsonType&, const std::optional<T>&) must not be
-        // noexcept: it calls T's to_json, which may throw (a user-defined
-        // to_json that reports an error, or std::bad_alloc for T =
-        // std::string/vector/json). Before the fix, this called
-        // std::terminate() instead of letting the exception propagate.
-        const std::optional<throwing_to_json_type> opt = throwing_to_json_type{};
-        CHECK_THROWS_WITH_AS(json(opt), "cannot serialize throwing_to_json_type", std::runtime_error&);
-
-        // the conversion is noexcept exactly when converting the contained value is
-        // (except with MSVC 2017, where it is never noexcept, see to_json.hpp)
-#if !defined(_MSC_VER) || defined(__clang__) || _MSC_VER >= 1920
-        static_assert(!std::is_nothrow_constructible<json, const std::optional<throwing_to_json_type>&>::value);
-        static_assert(std::is_nothrow_constructible<json, const std::optional<int>&>::value);
-#endif
-    }
-#endif
-}
-#endif
-
-#ifdef JSON_HAS_CPP_17
-    #undef JSON_HAS_CPP_17
-#endif
-
-#ifdef JSON_HAS_CPP_14
-    #undef JSON_HAS_CPP_14
-#endif
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-custom-array-type.cpp

@@ -9,7 +9,9 @@
 #include "doctest_compatibility.h"
 
 #include <nlohmann/json.hpp>
+using nlohmann::json;
 
+#include <cstdint>
 #include <deque>
 #include <map>
 #include <memory>
@@ -164,3 +166,47 @@ TEST_CASE("array type without at()")
     CHECK(j.at(no_at_json::json_pointer("/1")) == 2);
     CHECK_THROWS_AS(j.at(no_at_json::json_pointer("/3")), no_at_json::out_of_range);
 }
+
+TEST_CASE("regression test #5476 - array type without reserve()")
+{
+    // the capacity reserved for definite-length arrays must not require the
+    // array type to have a reserve() member function
+    SECTION("std::deque")
+    {
+        const auto j = deque_json::parse(R"({"a":[1,[2,3]],"b":[]})");
+        CHECK(j.dump() == R"({"a":[1,[2,3]],"b":[]})");
+
+        // the binary formats pass a definite length to start_array()
+        CHECK(deque_json::from_cbor(deque_json::to_cbor(j)) == j);
+        CHECK(deque_json::from_msgpack(deque_json::to_msgpack(j)) == j);
+        CHECK(deque_json::from_bon8(deque_json::to_bon8(j)) == j);
+
+        // parse() instantiates the callback parser as well, which reserves too
+        const auto with_callback = deque_json::parse(R"([1,2,3])", [](int /*depth*/, deque_json::parse_event_t /*event*/, deque_json& /*parsed*/) noexcept
+        {
+            return true;
+        });
+        CHECK(with_callback == deque_json({1, 2, 3}));
+    }
+
+    SECTION("std::vector still reserves")
+    {
+        json array = json::array();
+        for (int i = 0; i < 100; ++i)
+        {
+            array.push_back(i);
+        }
+
+        const auto j = json::from_cbor(json::to_cbor(array));
+        CHECK(j == array);
+        CHECK(j.get_ref<const json::array_t&>().capacity() >= 100);
+    }
+
+    SECTION("the reservation stays capped")
+    {
+        // CBOR array announcing 2^32-1 elements, but truncated right after the
+        // header: the input must be rejected without reserving that capacity
+        const std::vector<std::uint8_t> truncated = {0x9A, 0xFF, 0xFF, 0xFF, 0xFF};
+        CHECK(json::from_cbor(truncated, true, false).is_discarded());
+    }
+}

tests/src/unit-custom-base-class.cpp

@@ -44,10 +44,10 @@ TEST_CASE("JSON Node Metadata")
 {
     SECTION("type int")
     {
-        using json = json_with_metadata<int>;
-        json null;
-        auto obj   = json::object();
-        auto array = json::array();
+        using json_t = json_with_metadata<int>;
+        json_t null;
+        auto obj   = json_t::object();
+        auto array = json_t::array();
 
         null.metadata()  = 1;
         obj.metadata()   = 2;
@@ -61,8 +61,8 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("type vector<int>")
     {
-        using json = json_with_metadata<std::vector<int>>;
-        json value;
+        using json_t = json_with_metadata<std::vector<int>>;
+        json_t value;
         value.metadata().emplace_back(1);
         auto copy = value;
         value.metadata().emplace_back(2);
@@ -75,12 +75,12 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("copy ctor")
     {
-        using json = json_with_metadata<std::vector<int>>;
-        json value;
+        using json_t = json_with_metadata<std::vector<int>>;
+        json_t value;
         value.metadata().emplace_back(1);
         value.metadata().emplace_back(2);
 
-        json copy = value;
+        json_t copy = value;
 
         CHECK(copy.metadata().size()  == 2);
         CHECK(copy.metadata().at(0)   == 1);
@@ -95,12 +95,12 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("move ctor")
     {
-        using json = json_with_metadata<std::vector<int>>;
-        json value;
+        using json_t = json_with_metadata<std::vector<int>>;
+        json_t value;
         value.metadata().emplace_back(1);
         value.metadata().emplace_back(2);
 
-        const json moved = std::move(value);
+        const json_t moved = std::move(value);
 
         CHECK(moved.metadata().size()  == 2);
         CHECK(moved.metadata().at(0)   == 1);
@@ -108,12 +108,12 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("move assign")
     {
-        using json = json_with_metadata<std::vector<int>>;
-        json value;
+        using json_t = json_with_metadata<std::vector<int>>;
+        json_t value;
         value.metadata().emplace_back(1);
         value.metadata().emplace_back(2);
 
-        json moved;
+        json_t moved;
         moved = std::move(value);
 
         CHECK(moved.metadata().size()  == 2);
@@ -122,12 +122,12 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("copy assign")
     {
-        using json = json_with_metadata<std::vector<int>>;
-        json value;
+        using json_t = json_with_metadata<std::vector<int>>;
+        json_t value;
         value.metadata().emplace_back(1);
         value.metadata().emplace_back(2);
 
-        json copy;
+        json_t copy;
         copy = value;
 
         CHECK(copy.metadata().size()  == 2);
@@ -143,8 +143,8 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("type unique_ptr<int>")
     {
-        using json = json_with_metadata<std::unique_ptr<int>>;
-        json value;
+        using json_t = json_with_metadata<std::unique_ptr<int>>;
+        json_t value;
         value.metadata().reset(new int(42)); // NOLINT(cppcoreguidelines-owning-memory)
         auto moved = std::move(value);
 
@@ -153,12 +153,12 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("type vector<int> in json array")
     {
-        using json = json_with_metadata<std::vector<int>>;
-        json value;
+        using json_t = json_with_metadata<std::vector<int>>;
+        json_t value;
         value.metadata().emplace_back(1);
         value.metadata().emplace_back(2);
 
-        json const array(10, value);
+        json_t const array(10, value);
 
         CHECK(value.metadata().size() == 2);
         CHECK(value.metadata().at(0)  == 1);
@@ -173,10 +173,10 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("member swap")
     {
-        using json = json_with_metadata<int>;
-        json a = 1;
+        using json_t = json_with_metadata<int>;
+        json_t a = 1;
         a.metadata() = 100;
-        json b = 2;
+        json_t b = 2;
         b.metadata() = 200;
 
         a.swap(b);
@@ -188,10 +188,10 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("nonmember swap")
     {
-        using json = json_with_metadata<int>;
-        json a = 1;
+        using json_t = json_with_metadata<int>;
+        json_t a = 1;
         a.metadata() = 100;
-        json b = 2;
+        json_t b = 2;
         b.metadata() = 200;
 
         using std::swap;
@@ -204,10 +204,10 @@ TEST_CASE("JSON Node Metadata")
     }
     SECTION("std::swap")
     {
-        using json = json_with_metadata<int>;
-        json a = 1;
+        using json_t = json_with_metadata<int>;
+        json_t a = 1;
         a.metadata() = 100;
-        json b = 2;
+        json_t b = 2;
         b.metadata() = 200;
 
         std::swap(a, b);
@@ -221,15 +221,15 @@ TEST_CASE("JSON Node Metadata")
     {
         // std::sort mixes swap() with moves; each value's metadata must
         // travel with it, just as it does for copy, move, and assignment
-        using json = json_with_metadata<int>;
-        std::vector<json> values;
+        using json_t = json_with_metadata<int>;
+        std::vector<json_t> values;
         for (const int v :
                 {
                     5, 3, 9, 1, 7, 2, 8, 4, 6, 0, 15, 13, 19, 11, 17, 12, 18, 14, 16, 10,
                     25, 23, 29, 21, 27, 22, 28, 24, 26, 20, 35, 33
                 })
         {
-            json value = v;
+            json_t value = v;
             value.metadata() = v;
             values.push_back(value);
         }
@@ -267,19 +267,19 @@ template <class Ptr, class Fnc>
 void visitor_adaptor::do_visit(const Ptr& ptr, const Fnc& fnc) const
 {
     using value_t = nlohmann::detail::value_t;
-    const json_with_visitor_t& json = *static_cast<const json_with_visitor_t*>(this); // NOLINT(cppcoreguidelines-pro-type-static-cast-downcast)
-    switch (json.type())
+    const json_with_visitor_t& self = *static_cast<const json_with_visitor_t*>(this); // NOLINT(cppcoreguidelines-pro-type-static-cast-downcast)
+    switch (self.type())
     {
         case value_t::object:
-            for (const auto& entry : json.items())
+            for (const auto& entry : self.items())
             {
                 entry.value().do_visit(ptr / entry.key(), fnc);
             }
             break;
         case value_t::array:
-            for (std::size_t i = 0; i < json.size(); ++i)
+            for (std::size_t i = 0; i < self.size(); ++i)
             {
-                json.at(i).do_visit(ptr / std::to_string(i), fnc);
+                self.at(i).do_visit(ptr / std::to_string(i), fnc);
             }
             break;
         case value_t::discarded:
@@ -292,22 +292,22 @@ void visitor_adaptor::do_visit(const Ptr& ptr, const Fnc& fnc) const
         case value_t::number_float:
         case value_t::binary:
         default:
-            fnc(ptr, json);
+            fnc(ptr, self);
     }
 }
 
 TEST_CASE("JSON Visit Node")
 {
-    json_with_visitor_t json;
-    json["null"];
-    json["int"]  = -1;
-    json["uint"] = 1U;
-    json["float"] = 1.0;
-    json["boolean"] = true;
-    json["string"] = "string";
-    json["array"].push_back(0);
-    json["array"].push_back(1);
-    json["array"].push_back(json);
+    json_with_visitor_t root;
+    root["null"];
+    root["int"]  = -1;
+    root["uint"] = 1U;
+    root["float"] = 1.0;
+    root["boolean"] = true;
+    root["string"] = "string";
+    root["array"].push_back(0);
+    root["array"].push_back(1);
+    root["array"].push_back(root);
 
     std::set<std::string> expected
     {
@@ -330,7 +330,7 @@ TEST_CASE("JSON Visit Node")
         "/array/2/array/1 - number_integer - 1"
     };
 
-    json.visit(
+    root.visit(
             [&](const json_with_visitor_t::json_pointer & p,
                 const json_with_visitor_t& j)
     {
@@ -374,7 +374,7 @@ TEST_CASE("JSON Visit Node")
                 break;
         }
         str << " - "  << j.dump();
-        CHECK(json.at(p) == j);
+        CHECK(root.at(p) == j);
         INFO(str.str());
         CHECK(expected.count(str.str()) == 1);
         expected.erase(str.str());
@@ -404,22 +404,22 @@ using json_with_hidden_base_members = nlohmann::json::with_base_class_t<base_cla
 
 TEST_CASE("JSON Node as_base_class")
 {
-    using json = json_with_hidden_base_members;
+    using json_t = json_with_hidden_base_members;
 
-    static_assert(std::is_same<decltype(std::declval<json&>().as_base_class()), json::json_base_class_t&>::value, "");
-    static_assert(std::is_same<decltype(std::declval<const json&>().as_base_class()), const json::json_base_class_t&>::value, "");
-    static_assert(noexcept(std::declval<json&>().as_base_class()), "");
-    static_assert(noexcept(std::declval<const json&>().as_base_class()), "");
+    static_assert(std::is_same<decltype(std::declval<json_t&>().as_base_class()), json_t::json_base_class_t&>::value, "");
+    static_assert(std::is_same<decltype(std::declval<const json_t&>().as_base_class()), const json_t::json_base_class_t&>::value, "");
+    static_assert(noexcept(std::declval<json_t&>().as_base_class()), "");
+    static_assert(noexcept(std::declval<const json_t&>().as_base_class()), "");
 
     SECTION("non-const")
     {
-        json j = {1, 2, 3};
+        json_t j = {1, 2, 3};
 
         CHECK(std::string(j.type_name()) == "array");
         CHECK(j.size() == 3);
         CHECK(std::string(j.as_base_class().type_name()) == "custom type_name");
         CHECK(j.as_base_class().size() == 42);
-        CHECK(&j.as_base_class() == &static_cast<json::json_base_class_t&>(j));
+        CHECK(&j.as_base_class() == &static_cast<json_t::json_base_class_t&>(j));
 
         j.as_base_class().m_size = 7;
         CHECK(j.as_base_class().size() == 7);
@@ -428,13 +428,13 @@ TEST_CASE("JSON Node as_base_class")
 
     SECTION("const")
     {
-        const json j = {1, 2, 3};
+        const json_t j = {1, 2, 3};
 
         CHECK(std::string(j.type_name()) == "array");
         CHECK(j.size() == 3);
         CHECK(std::string(j.as_base_class().type_name()) == "custom type_name");
         CHECK(j.as_base_class().size() == 42);
-        CHECK(&j.as_base_class() == &static_cast<const json::json_base_class_t&>(j));
+        CHECK(&j.as_base_class() == &static_cast<const json_t::json_base_class_t&>(j));
     }
 }
 

tests/src/unit-custom-binary-type-cpp17.cpp

@@ -0,0 +1,111 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-custom-binary-type.cpp (binary types
+// whose value type is std::byte). It is kept in a separate translation unit so the (much
+// larger) unit-custom-binary-type.cpp is built for C++11 only and not rebuilt for every
+// C++ standard.
+
+#include "doctest_compatibility.h"
+
+// capture whether JSON_DISABLE_ENUM_SERIALIZATION was enabled on the command
+// line before including json.hpp, which #undefs it; std::byte is an enum, so
+// converting std::vector<std::byte> needs enum serialization
+#if defined(JSON_DISABLE_ENUM_SERIALIZATION) && (JSON_DISABLE_ENUM_SERIALIZATION == 1)
+    #define SKIP_TESTS_FOR_ENUM_SERIALIZATION
+#endif
+
+#include <nlohmann/json.hpp>
+
+#ifdef JSON_HAS_CPP_17
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+// a BinaryType whose value type is not an integer type at all
+using byte_binary_json = nlohmann::json::with_binary_t<std::vector<std::byte>>;
+
+TEST_CASE("binary type whose value type is not std::uint8_t (C++17)")
+{
+    SECTION("dumping a value type that is not an integer")
+    {
+        const std::vector<std::byte> bytes{std::byte{0}, std::byte{1}, std::byte{0xFF}};
+        CHECK(byte_binary_json::binary(bytes).dump() == R"({"bytes":[0,1,255],"subtype":null})");
+        CHECK(byte_binary_json::binary(bytes, 42).dump() == R"({"bytes":[0,1,255],"subtype":42})");
+        CHECK(byte_binary_json::binary({}).dump() == R"({"bytes":[],"subtype":null})");
+    }
+
+    SECTION("hashing and the binary formats")
+    {
+        const std::vector<std::byte> bytes{std::byte{0}, std::byte{1}, std::byte{0xFF}};
+        const auto j = byte_binary_json::binary(bytes);
+
+        CHECK(std::hash<byte_binary_json> {}(j) == std::hash<byte_binary_json> {}(j));
+        CHECK(byte_binary_json::from_cbor(byte_binary_json::to_cbor(j)) == j);
+        CHECK(byte_binary_json::from_msgpack(byte_binary_json::to_msgpack(j)) == j);
+
+        // UBJSON has no binary type, so binary values are written as an array
+        CHECK(byte_binary_json::from_ubjson(byte_binary_json::to_ubjson(j)) == byte_binary_json({0, 1, 255}));
+        // the same holds for BON8
+        CHECK(byte_binary_json::from_bon8(byte_binary_json::to_bon8(j)) == byte_binary_json({0, 1, 255}));
+    }
+}
+
+// moved from unit-regression3.cpp, so that file does not instantiate a second
+// basic_json specialization (MinGW section limit, see windows.yml)
+TEST_CASE("std::vector<std::byte> as binary type")
+{
+    SECTION("issue #4804: from_cbor incompatible with std::vector<std::byte> as binary_t")
+    {
+        const std::vector<std::uint8_t> data = {0x80};
+        const auto decoded = byte_binary_json::from_cbor(data);
+        CHECK((decoded == byte_binary_json::array()));
+    }
+
+#ifndef SKIP_TESTS_FOR_ENUM_SERIALIZATION
+    SECTION("discussion #4209 - custom BinaryType direct assignment and round-tripping")
+    {
+        // Test that assigning a custom BinaryType directly creates a binary value, not an array
+        const std::vector<std::byte> original{std::byte{1}, std::byte{2}, std::byte{3}};
+        const byte_binary_json j = original;
+        CHECK(j.is_binary());
+        CHECK(!j.is_array());
+
+        // Test round-tripping: extracting the binary value back as the custom container type
+        const auto extracted = j.get<std::vector<std::byte>>();
+        CHECK(extracted == original);
+
+        // Test that the default json alias behavior is unchanged: std::vector<uint8_t> -> array
+        const nlohmann::json default_json = std::vector<std::uint8_t> {1, 2, 3};
+        CHECK(default_json.is_array());
+        CHECK(!default_json.is_binary());
+    }
+
+    SECTION("discussion #4209 - custom BinaryType extraction from parsed array")
+    {
+        // Test that extracting a custom BinaryType from a parsed JSON array still works
+        // (not just from a binary-typed node)
+        const auto j = byte_binary_json::parse("[1,2,3]");
+        CHECK(j.is_array());
+        CHECK(!j.is_binary());
+
+        // Extracting as custom BinaryType should work from arrays
+        const auto extracted = j.get<std::vector<std::byte>>();
+        CHECK(extracted.size() == 3);
+        CHECK(extracted[0] == std::byte{1});
+        CHECK(extracted[1] == std::byte{2});
+        CHECK(extracted[2] == std::byte{3});
+    }
+#endif
+}
+
+#endif

tests/src/unit-custom-binary-type.cpp

@@ -8,13 +8,6 @@
 
 #include "doctest_compatibility.h"
 
-// capture whether JSON_DISABLE_ENUM_SERIALIZATION was enabled on the command
-// line before including json.hpp, which #undefs it; std::byte is an enum, so
-// converting std::vector<std::byte> needs enum serialization
-#if defined(JSON_DISABLE_ENUM_SERIALIZATION) && (JSON_DISABLE_ENUM_SERIALIZATION == 1)
-    #define SKIP_TESTS_FOR_ENUM_SERIALIZATION
-#endif
-
 #include <nlohmann/json.hpp>
 
 #include <cstdint>
@@ -24,10 +17,6 @@
 #include <string>
 #include <vector>
 
-#ifdef JSON_HAS_CPP_17
-    #include <cstddef>
-#endif
-
 namespace
 {
 
@@ -35,11 +24,6 @@ namespace
 // processed as the numbers 0..255
 using char_binary_json = nlohmann::json::with_binary_t<std::vector<char>>;
 
-#ifdef JSON_HAS_CPP_17
-    // a BinaryType whose value type is not an integer type at all
-    using byte_binary_json = nlohmann::json::with_binary_t<std::vector<std::byte>>;
-#endif
-
 } // namespace
 
 TEST_CASE("binary type whose value type is not std::uint8_t")
@@ -66,79 +50,4 @@ TEST_CASE("binary type whose value type is not std::uint8_t")
     {
         CHECK(nlohmann::json::binary({0, 1, 255}, 42).dump() == R"({"bytes":[0,1,255],"subtype":42})");
     }
-
-#ifdef JSON_HAS_CPP_17
-    SECTION("dumping a value type that is not an integer")
-    {
-        const std::vector<std::byte> bytes{std::byte{0}, std::byte{1}, std::byte{0xFF}};
-        CHECK(byte_binary_json::binary(bytes).dump() == R"({"bytes":[0,1,255],"subtype":null})");
-        CHECK(byte_binary_json::binary(bytes, 42).dump() == R"({"bytes":[0,1,255],"subtype":42})");
-        CHECK(byte_binary_json::binary({}).dump() == R"({"bytes":[],"subtype":null})");
-    }
-
-    SECTION("hashing and the binary formats")
-    {
-        const std::vector<std::byte> bytes{std::byte{0}, std::byte{1}, std::byte{0xFF}};
-        const auto j = byte_binary_json::binary(bytes);
-
-        CHECK(std::hash<byte_binary_json> {}(j) == std::hash<byte_binary_json> {}(j));
-        CHECK(byte_binary_json::from_cbor(byte_binary_json::to_cbor(j)) == j);
-        CHECK(byte_binary_json::from_msgpack(byte_binary_json::to_msgpack(j)) == j);
-
-        // UBJSON has no binary type, so binary values are written as an array
-        CHECK(byte_binary_json::from_ubjson(byte_binary_json::to_ubjson(j)) == byte_binary_json({0, 1, 255}));
-        // the same holds for BON8
-        CHECK(byte_binary_json::from_bon8(byte_binary_json::to_bon8(j)) == byte_binary_json({0, 1, 255}));
-    }
-#endif
 }
-
-#ifdef JSON_HAS_CPP_17
-// moved from unit-regression3.cpp, so that file does not instantiate a second
-// basic_json specialization (MinGW section limit, see windows.yml)
-TEST_CASE("std::vector<std::byte> as binary type")
-{
-    SECTION("issue #4804: from_cbor incompatible with std::vector<std::byte> as binary_t")
-    {
-        const std::vector<std::uint8_t> data = {0x80};
-        const auto decoded = byte_binary_json::from_cbor(data);
-        CHECK((decoded == byte_binary_json::array()));
-    }
-
-#ifndef SKIP_TESTS_FOR_ENUM_SERIALIZATION
-    SECTION("discussion #4209 - custom BinaryType direct assignment and round-tripping")
-    {
-        // Test that assigning a custom BinaryType directly creates a binary value, not an array
-        const std::vector<std::byte> original{std::byte{1}, std::byte{2}, std::byte{3}};
-        const byte_binary_json j = original;
-        CHECK(j.is_binary());
-        CHECK(!j.is_array());
-
-        // Test round-tripping: extracting the binary value back as the custom container type
-        const auto extracted = j.get<std::vector<std::byte>>();
-        CHECK(extracted == original);
-
-        // Test that the default json alias behavior is unchanged: std::vector<uint8_t> -> array
-        const nlohmann::json default_json = std::vector<std::uint8_t> {1, 2, 3};
-        CHECK(default_json.is_array());
-        CHECK(!default_json.is_binary());
-    }
-
-    SECTION("discussion #4209 - custom BinaryType extraction from parsed array")
-    {
-        // Test that extracting a custom BinaryType from a parsed JSON array still works
-        // (not just from a binary-typed node)
-        const auto j = byte_binary_json::parse("[1,2,3]");
-        CHECK(j.is_array());
-        CHECK(!j.is_binary());
-
-        // Extracting as custom BinaryType should work from arrays
-        const auto extracted = j.get<std::vector<std::byte>>();
-        CHECK(extracted.size() == 3);
-        CHECK(extracted[0] == std::byte{1});
-        CHECK(extracted[1] == std::byte{2});
-        CHECK(extracted[2] == std::byte{3});
-    }
-#endif
-}
-#endif

tests/src/unit-custom-number-types.cpp

@@ -0,0 +1,367 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#include <cstdint>
+#include <limits>
+#include <map>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#if JSON_HAS_THREE_WAY_COMPARISON
+    #include <compare>
+#endif
+
+namespace custom_number_types
+{
+
+// a signed integer of class type: std::is_signed is only true for arithmetic
+// types, so the library has to take the signedness from std::numeric_limits,
+// as it does for 128-bit and multiprecision integer classes such as
+// absl::int128 or boost::multiprecision::cpp_int
+class class_int
+{
+  public:
+    // trivial, like the 128-bit integer classes: the value is stored in a union
+    class_int() = default;
+
+    // implicit from built-in integers and explicit from floats, like absl::int128;
+    // non-template overloads (as absl::int128 has) because clang 3.4 and 3.5
+    // reject basic_json's defaulted data() for a number type with template
+    // constructors
+    class_int(int v) : value(v) {} // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+    class_int(unsigned v) : value(v) {} // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+    class_int(long v) : value(v) {} // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+    class_int(unsigned long v) : value(static_cast<std::int64_t>(v)) {} // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+    class_int(long long v) : value(v) {} // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+    class_int(unsigned long long v) : value(static_cast<std::int64_t>(v)) {} // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+    explicit class_int(double v) : value(static_cast<std::int64_t>(v)) {}
+
+    template<typename T, typename std::enable_if<std::is_arithmetic<T>::value, int>::type = 0>
+    explicit operator T() const
+    {
+        return static_cast<T>(value);
+    }
+
+    friend bool operator==(class_int lhs, class_int rhs)
+    {
+        return lhs.value == rhs.value;
+    }
+    friend bool operator!=(class_int lhs, class_int rhs)
+    {
+        return lhs.value != rhs.value;
+    }
+    friend bool operator<(class_int lhs, class_int rhs)
+    {
+        return lhs.value < rhs.value;
+    }
+#if JSON_HAS_THREE_WAY_COMPARISON
+    friend std::strong_ordering operator<=>(class_int lhs, class_int rhs) // *NOPAD*
+    {
+        return lhs.value <=> rhs.value; // *NOPAD*
+    }
+#endif
+
+  private:
+    std::int64_t value;
+};
+
+} // namespace custom_number_types
+
+using custom_number_types::class_int;
+
+namespace std
+{
+// only the members the library uses
+template<>
+class numeric_limits<class_int>
+{
+  public:
+    static constexpr bool is_signed = true;
+    static constexpr int digits = std::numeric_limits<std::int64_t>::digits;
+};
+} // namespace std
+
+namespace
+{
+
+using class_int_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, class_int, std::uint64_t, double>;
+
+class_int_json make_class_int(std::int64_t v)
+{
+    class_int_json j(class_int_json::value_t::number_integer);
+    j.get_ref<class_int&>() = class_int(v);
+    return j;
+}
+
+} // namespace
+
+// the serializer cannot print an integer of class type, so doctest must not
+// try when an assertion fails
+namespace doctest
+{
+template<>
+struct StringMaker<class_int_json>
+{
+    static String convert(const class_int_json& j)
+    {
+        return j.type_name();
+    }
+};
+} // namespace doctest
+
+// __int128 as number type needs the std::numeric_limits (for the range checks)
+// and std::is_integral (for the serializer) specializations, which libc++
+// always provides and libstdc++ only outside strict ISO modes; the MSVC
+// standard library has none
+#if defined(__SIZEOF_INT128__) && (defined(_LIBCPP_VERSION) || defined(__GLIBCXX_TYPE_INT_N_0))
+    #define JSON_TEST_INT128_NUMBER_TYPES 1
+#else
+    #define JSON_TEST_INT128_NUMBER_TYPES 0
+#endif
+
+#if JSON_TEST_INT128_NUMBER_TYPES
+namespace
+{
+
+// __extension__ keeps -Wpedantic from flagging the non-standard type
+__extension__ typedef __int128 int128; // NOLINT(modernize-use-using)
+__extension__ typedef unsigned __int128 uint128; // NOLINT(modernize-use-using)
+
+static_assert(std::numeric_limits<int128>::digits == 127 && std::numeric_limits<uint128>::digits == 128 &&
+              std::is_integral<int128>::value && std::is_integral<uint128>::value, // NOLINT(misc-redundant-expression)
+              "__int128 is not fully supported by the standard library");
+
+using wide_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, int128, uint128, double>;
+
+wide_json make_int(int128 v)
+{
+    wide_json j(wide_json::value_t::number_integer);
+    j.get_ref<int128&>() = v;
+    return j;
+}
+
+wide_json make_uint(uint128 v)
+{
+    wide_json j(wide_json::value_t::number_unsigned);
+    j.get_ref<uint128&>() = v;
+    return j;
+}
+
+wide_json wrap(const wide_json& value)
+{
+    wide_json o(wide_json::value_t::object);
+    o["v"] = value;
+    return o;
+}
+
+} // namespace
+
+TEST_CASE("custom number types: integers beyond 64 bits")
+{
+    using out_of_range = wide_json::out_of_range;
+
+    const int128 two_64 = static_cast<int128>(1) << 64;
+    const int128 two_100 = static_cast<int128>(1) << 100;
+    const int128 max_int64 = (std::numeric_limits<std::int64_t>::max)();
+    const int128 min_int64 = (std::numeric_limits<std::int64_t>::min)();
+    const uint128 max_uint64 = (std::numeric_limits<std::uint64_t>::max)();
+
+    // before the range checks, these values were silently truncated, e.g.,
+    // 2^100 was written as 0
+    const wide_json int_big = make_int(two_100);
+    const wide_json int_big_negative = make_int(-two_100);
+    const wide_json uint_big = make_uint(static_cast<uint128>(two_100));
+    std::vector<std::uint8_t> _;
+
+    SECTION("CBOR")
+    {
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_cbor(int_big), "[json.exception.out_of_range.407] integer number 1267650600228229401496703205376 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_cbor(int_big_negative), "[json.exception.out_of_range.407] integer number -1267650600228229401496703205376 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_cbor(uint_big), "[json.exception.out_of_range.407] integer number 1267650600228229401496703205376 cannot be represented by CBOR as it does not fit uint64", out_of_range&);
+        // the extremes of the 128-bit types, whose digits are produced without
+        // negating the minimum as a whole
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_cbor(make_int((std::numeric_limits<int128>::min)())), "[json.exception.out_of_range.407] integer number -170141183460469231731687303715884105728 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_cbor(make_int((std::numeric_limits<int128>::max)())), "[json.exception.out_of_range.407] integer number 170141183460469231731687303715884105727 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_cbor(make_uint((std::numeric_limits<uint128>::max)())), "[json.exception.out_of_range.407] integer number 340282366920938463463374607431768211455 cannot be represented by CBOR as it does not fit uint64", out_of_range&);
+        CHECK_THROWS_AS(_ = wide_json::to_cbor(make_int(two_64)), out_of_range&);
+        CHECK_THROWS_AS(_ = wide_json::to_cbor(make_int(-two_64 - 1)), out_of_range&);
+        CHECK_THROWS_AS(_ = wide_json::to_cbor(make_uint(static_cast<uint128>(max_uint64) + 1)), out_of_range&);
+
+        // CBOR's integers cover [-2^64, 2^64-1]
+        CHECK(wide_json::to_cbor(make_int(two_64 - 1)) == std::vector<std::uint8_t>({0x1B, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}));
+        CHECK(wide_json::to_cbor(make_int(-two_64)) == std::vector<std::uint8_t>({0x3B, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}));
+        CHECK(wide_json::to_cbor(make_uint(max_uint64)) == std::vector<std::uint8_t>({0x1B, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}));
+    }
+
+    SECTION("MessagePack")
+    {
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_msgpack(int_big), "[json.exception.out_of_range.407] integer number 1267650600228229401496703205376 cannot be represented by MessagePack as it does not fit [-2^63, 2^64-1]", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_msgpack(int_big_negative), "[json.exception.out_of_range.407] integer number -1267650600228229401496703205376 cannot be represented by MessagePack as it does not fit [-2^63, 2^64-1]", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_msgpack(uint_big), "[json.exception.out_of_range.407] integer number 1267650600228229401496703205376 cannot be represented by MessagePack as it does not fit uint64", out_of_range&);
+        CHECK_THROWS_AS(_ = wide_json::to_msgpack(make_int(two_64)), out_of_range&);
+        CHECK_THROWS_AS(_ = wide_json::to_msgpack(make_int(min_int64 - 1)), out_of_range&);
+
+        // MessagePack's integers cover [-2^63, 2^64-1]
+        CHECK(wide_json::to_msgpack(make_int(two_64 - 1)) == std::vector<std::uint8_t>({0xCF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}));
+        CHECK(wide_json::to_msgpack(make_int(min_int64)) == std::vector<std::uint8_t>({0xD3, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}));
+        CHECK(wide_json::to_msgpack(make_uint(max_uint64)) == std::vector<std::uint8_t>({0xCF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}));
+    }
+
+    SECTION("BSON")
+    {
+        // wrap() stores the value at /v, which JSON_DIAGNOSTICS adds to the message
+#if JSON_DIAGNOSTICS
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bson(wrap(int_big)), "[json.exception.out_of_range.407] (/v) integer number 1267650600228229401496703205376 cannot be represented by BSON as it does not fit int64", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bson(wrap(int_big_negative)), "[json.exception.out_of_range.407] (/v) integer number -1267650600228229401496703205376 cannot be represented by BSON as it does not fit int64", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bson(wrap(uint_big)), "[json.exception.out_of_range.407] (/v) integer number 1267650600228229401496703205376 cannot be represented by BSON as it does not fit uint64", out_of_range&);
+#else
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bson(wrap(int_big)), "[json.exception.out_of_range.407] integer number 1267650600228229401496703205376 cannot be represented by BSON as it does not fit int64", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bson(wrap(int_big_negative)), "[json.exception.out_of_range.407] integer number -1267650600228229401496703205376 cannot be represented by BSON as it does not fit int64", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bson(wrap(uint_big)), "[json.exception.out_of_range.407] integer number 1267650600228229401496703205376 cannot be represented by BSON as it does not fit uint64", out_of_range&);
+#endif
+        CHECK_THROWS_AS(_ = wide_json::to_bson(wrap(make_int(max_int64 + 1))), out_of_range&);
+        CHECK_THROWS_AS(_ = wide_json::to_bson(wrap(make_int(min_int64 - 1))), out_of_range&);
+
+        // nested values are checked as well, before anything is written
+        wide_json nested(wide_json::value_t::object);
+        nested["a"] = wide_json::array({wrap(int_big)});
+        std::vector<std::uint8_t> out;
+        CHECK_THROWS_AS(wide_json::to_bson(nested, out), out_of_range&);
+        CHECK(out.empty());
+
+        CHECK(wide_json::from_bson(wide_json::to_bson(wrap(make_int(max_int64)))) == wrap(make_int(max_int64)));
+        CHECK(wide_json::from_bson(wide_json::to_bson(wrap(make_int(min_int64)))) == wrap(make_int(min_int64)));
+        CHECK_NOTHROW(wide_json::to_bson(wrap(make_uint(max_uint64))));
+    }
+
+    SECTION("BON8")
+    {
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bon8(int_big), "[json.exception.out_of_range.407] integer number 1267650600228229401496703205376 cannot be represented by BON8 as it does not fit int64", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bon8(int_big_negative), "[json.exception.out_of_range.407] integer number -1267650600228229401496703205376 cannot be represented by BON8 as it does not fit int64", out_of_range&);
+        CHECK_THROWS_WITH_AS(_ = wide_json::to_bon8(uint_big), "[json.exception.out_of_range.407] integer number 1267650600228229401496703205376 cannot be represented by BON8 as it does not fit int64", out_of_range&);
+        CHECK_THROWS_AS(_ = wide_json::to_bon8(make_int(max_int64 + 1)), out_of_range&);
+        CHECK_THROWS_AS(_ = wide_json::to_bon8(make_int(min_int64 - 1)), out_of_range&);
+
+        CHECK(wide_json::from_bon8(wide_json::to_bon8(make_int(max_int64))) == make_int(max_int64));
+        CHECK(wide_json::from_bon8(wide_json::to_bon8(make_int(min_int64))) == make_int(min_int64));
+    }
+
+    SECTION("UBJSON and BJData")
+    {
+        // integers beyond 64 bits are written exactly as high-precision numbers
+        const std::string digits = "1267650600228229401496703205376";
+        std::vector<std::uint8_t> expected = {'H', 'i', static_cast<std::uint8_t>(digits.size())};
+        expected.insert(expected.end(), digits.begin(), digits.end());
+        CHECK(wide_json::to_ubjson(int_big) == expected);
+        CHECK(wide_json::to_ubjson(uint_big) == expected);
+        CHECK(wide_json::to_bjdata(int_big) == expected);
+        // BJData's uint64 marker 'M' was used for any unsigned value beyond
+        // int64, which truncated the ones beyond 64 bits
+        CHECK(wide_json::to_bjdata(uint_big) == expected);
+
+        // the uint64 marker is still used where the value fits
+        CHECK(wide_json::to_bjdata(make_uint(max_uint64)) == std::vector<std::uint8_t>({'M', 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}));
+
+        // the elements of an optimized container are written the same way;
+        // BJData does not allow 'H' as the type of an optimized container
+        std::vector<std::uint8_t> expected_ubjson_array = {'[', '$', 'H', '#', 'i', 2};
+        std::vector<std::uint8_t> expected_bjdata_array = {'[', '#', 'i', 2};
+        for (int i = 0; i < 2; ++i)
+        {
+            expected_ubjson_array.insert(expected_ubjson_array.end(), expected.begin() + 1, expected.end());
+            expected_bjdata_array.insert(expected_bjdata_array.end(), expected.begin(), expected.end());
+        }
+        CHECK(wide_json::to_ubjson(wide_json::array({uint_big, uint_big}), true, true) == expected_ubjson_array);
+        CHECK(wide_json::to_bjdata(wide_json::array({uint_big, uint_big}), true, true) == expected_bjdata_array);
+    }
+
+    SECTION("BJData ND-array")
+    {
+        // an ND-array element beyond 64 bits does not fit any dtype, so the
+        // annotated object is written as a plain object instead of truncating
+        // the element into range
+        wide_json element(wide_json::value_t::object);
+        element["_ArrayType_"] = "uint8";
+        element["_ArraySize_"] = wide_json::array({make_uint(2), make_uint(1)});
+        element["_ArrayData_"] = wide_json::array({make_uint(1), make_uint(static_cast<uint128>(two_64) + 1)});
+        const auto element_bytes = wide_json::to_bjdata(element, true, true);
+        REQUIRE(!element_bytes.empty());
+        CHECK(element_bytes[0] == '{');
+
+        wide_json signed_element = element;
+        signed_element["_ArrayType_"] = "int8";
+        signed_element["_ArrayData_"] = wide_json::array({make_int(1), make_int(-two_64 + 1)});
+        const auto signed_element_bytes = wide_json::to_bjdata(signed_element, true, true);
+        REQUIRE(!signed_element_bytes.empty());
+        CHECK(signed_element_bytes[0] == '{');
+
+        // the same for a dimension beyond 64 bits, which wrapped into a
+        // dimension matching the size of _ArrayData_ (here: 1)
+        wide_json dimension = element;
+        dimension["_ArraySize_"] = wide_json::array({make_uint(2), make_uint(static_cast<uint128>(two_64) + 1)});
+        dimension["_ArrayData_"] = wide_json::array({make_uint(1), make_uint(2)});
+        const auto dimension_bytes = wide_json::to_bjdata(dimension, true, true);
+        REQUIRE(!dimension_bytes.empty());
+        CHECK(dimension_bytes[0] == '{');
+
+        // in range, the ND-array is still written
+        wide_json fits = element;
+        fits["_ArrayData_"] = wide_json::array({make_uint(1), make_uint(2)});
+        const auto fits_bytes = wide_json::to_bjdata(fits, true, true);
+        REQUIRE(!fits_bytes.empty());
+        CHECK(fits_bytes[0] == '[');
+    }
+}
+
+#endif
+
+TEST_CASE("custom number types")
+{
+    SECTION("signed integer of class type compared with a float")
+    {
+        // std::is_signed is false for a class type, which made the comparison
+        // treat any float below zero as less than every integer
+        const class_int_json minus_five = make_class_int(-5);
+        const class_int_json minus_two = make_class_int(-2);
+        const class_int_json five = make_class_int(5);
+        const class_int_json minus_two_and_a_half = -2.5;
+        const class_int_json two_and_a_half = 2.5;
+        const class_int_json huge_negative = -1e30;
+        const class_int_json huge_positive = 1e30;
+
+        CHECK(minus_five < minus_two_and_a_half);
+        CHECK_FALSE(minus_two_and_a_half < minus_five);
+        CHECK(minus_two_and_a_half > minus_five);
+        CHECK(minus_five <= minus_two_and_a_half);
+        CHECK(minus_two_and_a_half >= minus_five);
+        CHECK(minus_five != minus_two_and_a_half);
+
+        CHECK(minus_two_and_a_half < minus_two);
+        CHECK_FALSE(minus_two < minus_two_and_a_half);
+
+        CHECK(two_and_a_half < five);
+        CHECK(minus_five < two_and_a_half);
+
+        // floats beyond the integer's range
+        CHECK(huge_negative < minus_five);
+        CHECK_FALSE(minus_five < huge_negative);
+        CHECK(five < huge_positive);
+        CHECK_FALSE(huge_positive < five);
+
+        // equality
+        const class_int_json minus_two_float = -2.0;
+        CHECK(minus_two == minus_two_float);
+        CHECK(minus_two_float == minus_two);
+    }
+
+}

tests/src/unit-deserialization-cpp20.cpp

@@ -0,0 +1,177 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++20-only part of unit-deserialization.cpp (char8_t support:
+// _json with char8_t literals and char8_t input). It is kept in a separate translation
+// unit so the (much larger) unit-deserialization.cpp is built for C++11 only and not
+// rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
+
+#ifdef JSON_HAS_CPP_20
+#include <string>
+#include <vector>
+
+#if defined(__cpp_char8_t) && (__cpp_char8_t >= 201811L)
+namespace
+{
+// copy of SaxEventLogger from unit-deserialization.cpp (renamed to avoid clashes in unity builds)
+struct SaxEventLoggerChar8 : public nlohmann::json_sax<json>
+{
+    bool null() override
+    {
+        events.emplace_back("null()");
+        return true;
+    }
+
+    bool boolean(bool val) override
+    {
+        events.emplace_back(val ? "boolean(true)" : "boolean(false)");
+        return true;
+    }
+
+    bool number_integer(json::number_integer_t val) override
+    {
+        events.push_back("number_integer(" + std::to_string(val) + ")");
+        return true;
+    }
+
+    bool number_unsigned(json::number_unsigned_t val) override
+    {
+        events.push_back("number_unsigned(" + std::to_string(val) + ")");
+        return true;
+    }
+
+    bool number_float(json::number_float_t /*val*/, const std::string& s) override
+    {
+        events.push_back("number_float(" + s + ")");
+        return true;
+    }
+
+    bool string(std::string& val) override
+    {
+        events.push_back("string(" + val + ")");
+        return true;
+    }
+
+    bool binary(json::binary_t& val) override
+    {
+        std::string binary_contents = "binary(";
+        std::string comma_space;
+        for (auto b : val)
+        {
+            binary_contents.append(comma_space);
+            binary_contents.append(std::to_string(static_cast<int>(b)));
+            comma_space = ", ";
+        }
+        binary_contents.append(")");
+        events.push_back(binary_contents);
+        return true;
+    }
+
+    bool start_object(std::size_t elements) override
+    {
+        if (elements == (std::numeric_limits<std::size_t>::max)())
+        {
+            events.emplace_back("start_object()");
+        }
+        else
+        {
+            events.push_back("start_object(" + std::to_string(elements) + ")");
+        }
+        return true;
+    }
+
+    bool key(std::string& val) override
+    {
+        events.push_back("key(" + val + ")");
+        return true;
+    }
+
+    bool end_object() override
+    {
+        events.emplace_back("end_object()");
+        return true;
+    }
+
+    bool start_array(std::size_t elements) override
+    {
+        if (elements == (std::numeric_limits<std::size_t>::max)())
+        {
+            events.emplace_back("start_array()");
+        }
+        else
+        {
+            events.push_back("start_array(" + std::to_string(elements) + ")");
+        }
+        return true;
+    }
+
+    bool end_array() override
+    {
+        events.emplace_back("end_array()");
+        return true;
+    }
+
+    bool parse_error(std::size_t position, const std::string& /*last_token*/, const json::exception& /*ex*/) override
+    {
+        events.push_back("parse_error(" + std::to_string(position) + ")");
+        return false;
+    }
+
+    std::vector<std::string> events {}; // NOLINT(readability-redundant-member-init)
+};
+} // namespace
+
+TEST_CASE_TEMPLATE("deserialization of different character types (ASCII) (C++20)", T, char8_t) // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
+{
+    std::vector<T> const v = {'t', 'r', 'u', 'e'};
+    CHECK(json::parse(v) == json(true));
+    CHECK(json::accept(v));
+
+    SaxEventLoggerChar8 l;
+    CHECK(json::sax_parse(v, &l));
+    CHECK(l.events.size() == 1);
+    CHECK(l.events == std::vector<std::string>({"boolean(true)"}));
+}
+#endif
+
+TEST_CASE("deserialization (C++20)")
+{
+#if defined(__cpp_char8_t)
+    SECTION("Using _json with char8_t literals #4945")
+    {
+        // Regular narrow string literal
+        const auto j1 = R"({"key": "value", "num": 42})"_json;
+        CHECK(j1["key"] == "value");
+        CHECK(j1["num"] == 42);
+
+        // UTF-8 prefixed literal (C++20 and later); the emoji is written as a
+        // \U escape rather than a raw multibyte character so this does not
+        // depend on the compiler's source-file encoding (e.g., MSVC without
+        // /utf-8, or classic ICC, which does not encode non-ASCII narrow
+        // string literals as UTF-8 - compare against a \x-escaped expectation
+        // for the same reason)
+        const auto j2 = u8"{\"emoji\": \"\U0001F600\", \"msg\": \"hello\"}"_json;
+        CHECK(j2["emoji"] == "\xF0\x9F\x98\x80");
+        CHECK(j2["msg"] == "hello");
+
+        const auto j3 = u8R"({"key": "value", "num": 42})"_json;
+        CHECK(j3["key"] == "value");
+        CHECK(j3["num"] == 42);
+    }
+#endif
+}
+
+#endif

tests/src/unit-deserialization.cpp

@@ -1167,44 +1167,9 @@ TEST_CASE("deserialization")
             CHECK(line == "foo");
         }
     }
-
-    // build with C++20
-    // JSON_HAS_CPP_20
-#if defined(__cpp_char8_t)
-    SECTION("Using _json with char8_t literals #4945")
-    {
-        // Regular narrow string literal
-        const auto j1 = R"({"key": "value", "num": 42})"_json;
-        CHECK(j1["key"] == "value");
-        CHECK(j1["num"] == 42);
-
-        // UTF-8 prefixed literal (C++20 and later); the emoji is written as a
-        // \U escape rather than a raw multibyte character so this does not
-        // depend on the compiler's source-file encoding (e.g., MSVC without
-        // /utf-8, or classic ICC, which does not encode non-ASCII narrow
-        // string literals as UTF-8 - compare against a \x-escaped expectation
-        // for the same reason)
-        const auto j2 = u8"{\"emoji\": \"\U0001F600\", \"msg\": \"hello\"}"_json;
-        CHECK(j2["emoji"] == "\xF0\x9F\x98\x80");
-        CHECK(j2["msg"] == "hello");
-
-        const auto j3 = u8R"({"key": "value", "num": 42})"_json;
-        CHECK(j3["key"] == "value");
-        CHECK(j3["num"] == 42);
-    }
-#endif
 }
 
-// select the types to test - char8_t is only available since C++20 if and only
-// if __cpp_char8_t is defined.
-#define TYPE_LIST(...) __VA_ARGS__
-#if defined(__cpp_char8_t) && (__cpp_char8_t >= 201811L)
-    #define ASCII_TYPES TYPE_LIST(char, wchar_t, char16_t, char32_t, char8_t)
-#else
-    #define ASCII_TYPES TYPE_LIST(char, wchar_t, char16_t, char32_t)
-#endif
-
-TEST_CASE_TEMPLATE("deserialization of different character types (ASCII)", T, ASCII_TYPES) // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
+TEST_CASE_TEMPLATE("deserialization of different character types (ASCII)", T, char, wchar_t, char16_t, char32_t) // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
 {
     std::vector<T> const v = {'t', 'r', 'u', 'e'};
     CHECK(json::parse(v) == json(true));

tests/src/unit-enum_keyed_maps_default.cpp

@@ -27,6 +27,9 @@ using nlohmann::json;
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
 DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
 
+// the types live in a namespace, because the file may be compiled together with other test files
+namespace enum_keyed_maps_default
+{
 enum class cards {kreuz, pik, herz, karo};
 
 // NOLINTNEXTLINE(misc-use-internal-linkage,misc-const-correctness,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
@@ -65,6 +68,7 @@ NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(strict_cards,
     {strict_cards::herz, "herz"},
     {strict_cards::karo, "karo"}
 })
+} // namespace enum_keyed_maps_default
 
 namespace
 {
@@ -82,6 +86,15 @@ struct enum_hash
 // see unit-enum_keyed_maps.cpp for JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS=1
 TEST_CASE("maps with enum keys")
 {
+    // block-scope using-declarations take precedence over same-named types of other files in the same translation unit
+    using enum_keyed_maps_default::cards;
+    using enum_keyed_maps_default::strict_cards;
+    using enum_keyed_maps_default::TaskState;
+    using enum_keyed_maps_default::TS_COMPLETED;
+    using enum_keyed_maps_default::TS_INVALID;
+    using enum_keyed_maps_default::TS_RUNNING;
+    using enum_keyed_maps_default::TS_STOPPED;
+
     using task_map = std::map<TaskState, std::string>;
     using task_umap = std::unordered_map<TaskState, std::string, enum_hash>;
     using task_gmap = std::map<TaskState, std::string, std::greater<TaskState>>;

tests/src/unit-explicit_instantiation.cpp

@@ -6,11 +6,11 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// The regression below only showed on C++17, so build this file for every
-// standard like the other regression tests:
-// JSON_HAS_CPP_17 JSON_HAS_CPP_20 (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. The regression below only
+// showed on C++17, and the whole file is just this explicit instantiation, so it
+// cannot be split: build this file for every standard like the other regression tests:
+// JSON_HAS_CPP_17 JSON_HAS_CPP_20 (do not remove)
 
 #include "doctest_compatibility.h"
 

tests/src/unit-guard-page.cpp

@@ -0,0 +1,503 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// The fast paths for contiguous input (bulk string scanning, SWAR searches,
+// UTF-8 prefix validation, the 8-digit number loop, the 8-byte indentation
+// skip, the BSON/BON8 bulk string readers) load several bytes at a time. This
+// test places every input directly in front of (or directly behind) an
+// inaccessible page, so reading even one byte outside the input crashes, with
+// or without sanitizers. ASan alone misses over-reads into the slack of the
+// same heap allocation.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#include <algorithm>
+#include <cstdint>
+#include <cstring>
+#include <string>
+#include <utility>
+#include <vector>
+
+#if defined(_WIN32)
+    #define JSON_TEST_GUARD_PAGE_WINDOWS
+    #ifndef NOMINMAX
+        #define NOMINMAX
+    #endif
+    #ifndef WIN32_LEAN_AND_MEAN
+        #define WIN32_LEAN_AND_MEAN
+    #endif
+    #include <windows.h>
+#elif defined(__unix__) || defined(__APPLE__)
+    #define JSON_TEST_GUARD_PAGE_POSIX
+    #include <sys/mman.h>
+    #include <unistd.h>
+#endif
+
+#if defined(JSON_TEST_GUARD_PAGE_WINDOWS) || defined(JSON_TEST_GUARD_PAGE_POSIX)
+
+namespace
+{
+// Three pages: an inaccessible one, a readable one, an inaccessible one. An
+// input is copied either to the end of the middle page (any read past its last
+// byte faults) or to its start (any read before its first byte faults).
+class guarded_buffer
+{
+  public:
+    guarded_buffer()
+        : page(page_size())
+    {
+#if defined(JSON_TEST_GUARD_PAGE_WINDOWS)
+        void* const p = VirtualAlloc(nullptr, 3 * page, MEM_RESERVE | MEM_COMMIT, PAGE_NOACCESS);
+        REQUIRE(p != nullptr);
+        base = static_cast<unsigned char*>(p);
+        DWORD old_protect = 0;
+        REQUIRE(VirtualProtect(base + page, page, PAGE_READWRITE, &old_protect) != 0);
+#else
+        void* const p = mmap(nullptr, 3 * page, PROT_NONE, MAP_PRIVATE | MAP_ANON, -1, 0);
+        REQUIRE(p != MAP_FAILED);
+        base = static_cast<unsigned char*>(p);
+        REQUIRE(mprotect(base + page, page, PROT_READ | PROT_WRITE) == 0);
+#endif
+    }
+
+    ~guarded_buffer()
+    {
+#if defined(JSON_TEST_GUARD_PAGE_WINDOWS)
+        VirtualFree(base, 0, MEM_RELEASE);
+#else
+        munmap(base, 3 * page);
+#endif
+    }
+
+    guarded_buffer(const guarded_buffer&) = delete;
+    guarded_buffer& operator=(const guarded_buffer&) = delete;
+    guarded_buffer(guarded_buffer&&) = delete;
+    guarded_buffer& operator=(guarded_buffer&&) = delete;
+
+    // copies @a input so its last byte is the last readable byte
+    const std::uint8_t* at_end(const std::string& input)
+    {
+        REQUIRE(input.size() <= page);
+        unsigned char* const first = base + (2 * page) - input.size();
+        std::transform(input.begin(), input.end(), first, [](const char c)
+        {
+            return static_cast<unsigned char>(c);
+        });
+        return first;
+    }
+
+    // copies @a input so its first byte is the first readable byte
+    const std::uint8_t* at_start(const std::string& input)
+    {
+        REQUIRE(input.size() <= page);
+        unsigned char* const first = base + page;
+        std::transform(input.begin(), input.end(), first, [](const char c)
+        {
+            return static_cast<unsigned char>(c);
+        });
+        return first;
+    }
+
+  private:
+    static std::size_t page_size()
+    {
+#if defined(JSON_TEST_GUARD_PAGE_WINDOWS)
+        SYSTEM_INFO info;
+        GetSystemInfo(&info);
+        return static_cast<std::size_t>(info.dwPageSize);
+#else
+        return static_cast<std::size_t>(sysconf(_SC_PAGESIZE));
+#endif
+    }
+
+    std::size_t page = 0;
+    unsigned char* base = nullptr;
+};
+
+// the serialization of the result of @a f, or the exception message; @a f
+// takes the allow_exceptions flag, which is false without exceptions, where
+// an error yields a discarded value instead
+template<typename F>
+std::string outcome(F f)
+{
+#if defined(JSON_NOEXCEPTION)
+    return "value: " + f(false).dump(-1, ' ', false, json::error_handler_t::keep);
+#else
+    try
+    {
+        return "value: " + f(true).dump(-1, ' ', false, json::error_handler_t::keep);
+    }
+    catch (const json::exception& e)
+    {
+        return std::string("error: ") + e.what();
+    }
+#endif
+}
+
+// a SAX handler that only counts events, so sax_parse runs without a DOM
+struct counting_sax : nlohmann::json_sax<json>
+{
+    std::size_t events = 0;
+    std::string error{}; // NOLINT(readability-redundant-member-init)
+
+    bool null() override
+    {
+        return count();
+    }
+    bool boolean(bool /*val*/) override
+    {
+        return count();
+    }
+    bool number_integer(number_integer_t /*val*/) override
+    {
+        return count();
+    }
+    bool number_unsigned(number_unsigned_t /*val*/) override
+    {
+        return count();
+    }
+    bool number_float(number_float_t /*val*/, const string_t& /*s*/) override
+    {
+        return count();
+    }
+    bool string(string_t& /*val*/) override
+    {
+        return count();
+    }
+    bool binary(binary_t& /*val*/) override
+    {
+        return count();
+    }
+    bool start_object(std::size_t /*elements*/) override
+    {
+        return count();
+    }
+    bool key(string_t& /*val*/) override
+    {
+        return count();
+    }
+    bool end_object() override
+    {
+        return count();
+    }
+    bool start_array(std::size_t /*elements*/) override
+    {
+        return count();
+    }
+    bool end_array() override
+    {
+        return count();
+    }
+    bool parse_error(std::size_t position, const std::string& /*last_token*/, const nlohmann::detail::exception& ex) override
+    {
+        error = std::to_string(position) + " " + ex.what();
+        return false;
+    }
+
+  private:
+    bool count()
+    {
+        ++events;
+        return true;
+    }
+};
+
+// all prefixes of each seed, so the input ends at every byte of it
+std::vector<std::string> all_prefixes(const std::vector<std::string>& seeds)
+{
+    std::vector<std::string> result;
+    for (const auto& seed : seeds)
+    {
+        for (std::size_t n = 0; n <= seed.size(); ++n)
+        {
+            result.push_back(seed.substr(0, n));
+        }
+    }
+    return result;
+}
+
+std::vector<std::string> json_seeds()
+{
+    std::vector<std::string> seeds =
+    {
+        // string runs: ASCII, escapes, \u escapes, surrogate pairs
+        "\"abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ\"",
+        R"("ab\ncd\"ef\\gh\u00e4ij\uD83D\uDE00kl\u12")",
+        // 2-, 3- and 4-byte UTF-8 sequences, then invalid UTF-8
+        std::string("\"\xC3\xA4\xE2\x82\xAC\xF0\x9F\x98\x80\xC3\xA4\xE2\x82\xAC\xF0\x9F\x98\x80") + "abcdefgh\"",
+        "\"abcdefgh\xC3\x28 \xED\xA0\x80 \xF4\x90\x80\x80 \xF8 \xC0\xAF\"",
+        std::string("\"abcdefg\x01") + "abcdefgh\"",
+        // numbers, so the 8-digit loop runs up to the end
+        "12345678",
+        "1234567890123456",
+        "12345678901234567890123",
+        "-0.000000001234567890123456789",
+        "3.14159265358979323846264338327950288e+100",
+        "-1.2345678901234567e-12",
+        "1E-5",
+        "[1.5,2e10,-3E-5,0.000000001,123456789012345678]",
+        // literals
+        "[true,false,null]",
+        // comments (only parsed with ignore_comments)
+        "/* comment */ [1, // line comment\n 2] /**/",
+        // a pretty-printed document
+        json::parse(R"({"a": [1, 2.5, "three", {"b": null, "c": [true, false]}], "d": {"e": {"f": {"g": "h"}}}})").dump(4),
+        json::parse(R"({"a": [1, 2.5, "three", {"b": null, "c": [true, false]}], "d": {"e": {"f": {"g": "h"}}}})").dump(8, '\t'),
+    };
+
+    // indentation runs of every length, with and without a closing bracket
+    for (std::size_t n = 0; n <= 20; ++n)
+    {
+        seeds.push_back("[\n" + std::string(n, ' ') + "1,\n" + std::string(n, ' ') + "2\n]");
+    }
+
+    return all_prefixes(seeds);
+}
+
+// a value whose binary serializations contain long strings and keys, numbers
+// of every width, and nested containers
+json binary_document()
+{
+    json j;
+    j["a key that is longer than eight bytes"] = "a string value that is longer than eight bytes";
+    j["\xC3\xA4\xE2\x82\xAC\xF0\x9F\x98\x80"] = "\xF0\x9F\x98\x80\xE2\x82\xAC\xC3\xA4 utf-8 0123456789";
+    j["numbers"] = {0, 1, -1, 255, -256, 65536, -2147483649LL, 4294967296ULL, 1.5, -2.25e100};
+    j["nested"] = {{"x", {{"y", {true, false, nullptr}}}}};
+    j["s"] = "";
+    return j;
+}
+
+// readers for check_binary(), callable with a container or an iterator range
+// followed by the allow_exceptions flag
+#define JSON_TEST_GUARD_PAGE_READER(name, from)                                     \
+    struct name                                                                     \
+    {                                                                               \
+        template<typename... Args>                                                  \
+        json operator()(bool allow_exceptions, Args&& ... args) const               \
+        {                                                                           \
+            return json::from(std::forward<Args>(args)..., true, allow_exceptions); \
+        }                                                                           \
+    };
+JSON_TEST_GUARD_PAGE_READER(read_bson, from_bson)
+JSON_TEST_GUARD_PAGE_READER(read_bon8, from_bon8)
+JSON_TEST_GUARD_PAGE_READER(read_cbor, from_cbor)
+JSON_TEST_GUARD_PAGE_READER(read_msgpack, from_msgpack)
+JSON_TEST_GUARD_PAGE_READER(read_ubjson, from_ubjson)
+JSON_TEST_GUARD_PAGE_READER(read_bjdata, from_bjdata)
+#undef JSON_TEST_GUARD_PAGE_READER
+
+// @a bytes as characters (explicitly converted: the iterator-range constructor
+// would convert implicitly, which -fsanitize=integer reports)
+std::string as_chars(const std::vector<std::uint8_t>& bytes)
+{
+    std::string result;
+    result.reserve(bytes.size());
+    for (const std::uint8_t b : bytes)
+    {
+        result.push_back(static_cast<char>(b));
+    }
+    return result;
+}
+
+// reads every prefix of @a bytes at both ends of the guarded page and from a
+// std::string, which must give the same outcome
+template<typename Reader>
+void check_binary(guarded_buffer& guard, const std::vector<std::uint8_t>& bytes, Reader parse)
+{
+    for (const auto& input : all_prefixes({as_chars(bytes)}))
+    {
+        CAPTURE(input)
+        const std::string expected = outcome([&](bool allow)
+        {
+            return parse(allow, input);
+        });
+        for (int placement = 0; placement < 2; ++placement)
+        {
+            const std::uint8_t* const first = placement == 0 ? guard.at_end(input) : guard.at_start(input);
+            CHECK(outcome([&](bool allow)
+            {
+                return parse(allow, first, first + input.size());
+            }) == expected);
+        }
+    }
+}
+
+// parse_float_native() at both ends of the guarded page, compared bit for bit
+// with converting the same token from a std::string
+template<typename FloatType>
+std::uint64_t float_bits(FloatType value)
+{
+    static_assert(sizeof(FloatType) <= sizeof(std::uint64_t), "unsupported floating-point width");
+    std::uint64_t bits = 0;
+    std::memcpy(&bits, &value, sizeof(value));
+    return bits;
+}
+
+template<typename FloatType>
+void check_float_native(guarded_buffer& guard, const std::string& token)
+{
+    const auto decimal_point = token.find('.');
+    const auto exponent = token.find_first_of("eE");
+    const auto mantissa_end = exponent == std::string::npos ? token.size() : exponent;
+    const auto expected = nlohmann::detail::parse_float_native<FloatType>(token.data(), token.data() + token.size(), decimal_point, mantissa_end);
+    for (int placement = 0; placement < 2; ++placement)
+    {
+        const char* const first = reinterpret_cast<const char*>(placement == 0 ? guard.at_end(token) : guard.at_start(token));
+        const auto value = nlohmann::detail::parse_float_native<FloatType>(first, first + token.size(), decimal_point, mantissa_end);
+        CHECK(float_bits(value) == float_bits(expected));
+    }
+}
+} // namespace
+
+TEST_CASE("guard page")
+{
+    guarded_buffer guard;
+
+    SECTION("string scanning")
+    {
+        // fills that drive each function to a different stop
+        const std::vector<std::string> fills =
+        {
+            std::string(64, 'a'),
+            std::string(64, '"'),
+            std::string(64, '\\'),
+            std::string(64, '\x01'),
+            std::string(64, '\x7F'),
+            std::string(64, '\xFF'),
+        };
+        std::vector<std::string> patterns = fills;
+        {
+            std::string utf8;
+            while (utf8.size() < 64)
+            {
+                utf8 += "\xC3\xA4\xE2\x82\xAC\xF0\x9F\x98\x80" "ab";
+            }
+            patterns.push_back(utf8);
+        }
+        {
+            std::string bytes;
+            for (unsigned int i = 0; i < 64; ++i)
+            {
+                bytes.push_back(static_cast<char>((i * 37u) & 0xFFu));
+            }
+            patterns.push_back(bytes);
+        }
+
+        for (const auto& pattern : patterns)
+        {
+            for (std::size_t n = 0; n <= 64; ++n)
+            {
+                const std::string input = pattern.substr(0, n);
+                for (int placement = 0; placement < 2; ++placement)
+                {
+                    const unsigned char* const p = placement == 0 ? guard.at_end(input) : guard.at_start(input);
+
+                    CHECK(nlohmann::detail::find_string_special(p, n) <= n);
+                    CHECK(nlohmann::detail::find_ascii_copyable_run(p, n) <= n);
+                    CHECK(nlohmann::detail::valid_utf8_prefix(p, n) <= n);
+#if defined(JSON_USE_SIMDUTF) && defined(JSON_HAS_CPP_17)
+                    CHECK(nlohmann::detail::find_string_delimiter(p, n) <= n);
+#endif
+                    CHECK(nlohmann::detail::string_bulk_run(p, n) <= n);
+                    CHECK(nlohmann::detail::scalar_string_bulk_run(p, n) <= n);
+                    for (std::size_t i = 0; i < n; ++i)
+                    {
+                        CHECK(nlohmann::detail::validate_one_utf8(p + i, n - i) <= n - i);
+                    }
+                }
+            }
+        }
+    }
+
+    SECTION("float conversion")
+    {
+        // the lexer converts floats from its token buffer, so the JSON section
+        // below never places a float token at the page boundary; this calls the
+        // conversion directly: the 8-digit loops of parse_float_significand()
+        // and digit_comparison() read up to the end of the token
+        const std::vector<std::string> tokens = all_prefixes(
+        {
+            "1234567812345678123456781234567812345678",
+            "-1.2345678901234567890123456789e-123",
+            "0.00000000000000000000000000000000000001",
+            "98765432.10987654",
+            "123456789012345678901234567890e-30",
+            // halfway between 1 and the next double, then just above it
+            "1.00000000000000011102230246251565404236316680908203125",
+            "1.000000000000000111022302462515654042363166809082031250000000001",
+        });
+        for (const auto& token : tokens)
+        {
+            // only complete number tokens: the conversion expects validated input
+            if (!token.empty() && json::accept(token))
+            {
+                CAPTURE(token)
+                check_float_native<double>(guard, token);
+                check_float_native<float>(guard, token);
+            }
+        }
+    }
+
+    SECTION("JSON")
+    {
+        for (const auto& input : json_seeds())
+        {
+            CAPTURE(input)
+            const std::string expected = outcome([&](bool allow)
+            {
+                return json::parse(input, nullptr, allow);
+            });
+            const std::string expected_comments = outcome([&](bool allow)
+            {
+                return json::parse(input, nullptr, allow, true);
+            });
+            const bool expected_accept = json::accept(input);
+            counting_sax expected_sax;
+            const bool expected_sax_result = json::sax_parse(input, &expected_sax);
+
+            for (int placement = 0; placement < 2; ++placement)
+            {
+                const std::uint8_t* const first = placement == 0 ? guard.at_end(input) : guard.at_start(input);
+                const std::uint8_t* const last = first + input.size();
+
+                CHECK(outcome([&](bool allow)
+                {
+                    return json::parse(first, last, nullptr, allow);
+                }) == expected);
+                CHECK(outcome([&](bool allow)
+                {
+                    return json::parse(first, last, nullptr, allow, true);
+                }) == expected_comments);
+                CHECK(json::accept(first, last) == expected_accept);
+
+                counting_sax sax;
+                CHECK(json::sax_parse(first, last, &sax) == expected_sax_result);
+                CHECK(sax.events == expected_sax.events);
+                CHECK(sax.error == expected_sax.error);
+            }
+        }
+    }
+
+    SECTION("binary formats")
+    {
+        // every prefix ends inside a value, e.g., inside a BSON key or BON8
+        // string without its terminator
+        const json j = binary_document();
+        check_binary(guard, json::to_bson(j), read_bson());
+        check_binary(guard, json::to_bon8(j), read_bon8());
+        check_binary(guard, json::to_cbor(j), read_cbor());
+        check_binary(guard, json::to_msgpack(j), read_msgpack());
+        check_binary(guard, json::to_ubjson(j), read_ubjson());
+        check_binary(guard, json::to_bjdata(j), read_bjdata());
+    }
+}
+
+#endif

tests/src/unit-iterators2-cpp20.cpp

@@ -0,0 +1,122 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++20-only part of unit-iterators2.cpp (iterators and
+// std::ranges). It is kept in a separate translation unit so the (much larger) unit-
+// iterators2.cpp is built for C++11 only and not rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#ifdef JSON_HAS_CPP_20
+#include <iterator>
+#include <string>
+#include <string_view>
+#include <type_traits>
+#include <utility>
+
+#if JSON_HAS_RANGES
+    #include <algorithm>
+    #include <ranges>
+#endif
+
+TEST_CASE("iterators 2 (C++20)")
+{
+#if JSON_HAS_RANGES
+    SECTION("ranges")
+    {
+        SECTION("concepts")
+        {
+            using nlohmann::detail::iteration_proxy_value;
+            CHECK(std::bidirectional_iterator<json::iterator>);
+            CHECK(std::input_iterator<iteration_proxy_value<json::iterator>>);
+
+            CHECK(std::is_same<json::iterator, std::ranges::iterator_t<json>>::value);
+            CHECK(std::ranges::bidirectional_range<json>);
+
+            using nlohmann::detail::iteration_proxy;
+            using items_type = decltype(std::declval<json&>().items());
+            CHECK(std::is_same<items_type, iteration_proxy<json::iterator>>::value);
+            CHECK(std::is_same<iteration_proxy_value<json::iterator>, std::ranges::iterator_t<items_type>>::value);
+            CHECK(std::ranges::input_range<items_type>);
+        }
+
+        SECTION("algorithms")
+        {
+            SECTION("copy")
+            {
+                json j{"foo", "bar"};
+                auto j_copied = json::array();
+
+                std::ranges::copy(j, std::back_inserter(j_copied));
+
+                CHECK(j == j_copied);
+            }
+
+            SECTION("find_if")
+            {
+                json j{1, 3, 2, 4};
+                auto j_even = json::array();
+
+#if JSON_USE_IMPLICIT_CONVERSIONS
+                auto it = std::ranges::find_if(j, [](int v) noexcept
+                {
+                    return (v % 2) == 0;
+                });
+#else
+                auto it = std::ranges::find_if(j, [](const json & j) noexcept
+                {
+                    int v;
+                    j.get_to(v);
+                    return (v % 2) == 0;
+                });
+#endif
+
+                CHECK(*it == 2);
+            }
+        }
+
+        SECTION("views")
+        {
+            SECTION("reverse")
+            {
+                json j{1, 2, 3, 4, 5};
+                json j_expected{5, 4, 3, 2, 1};
+
+                auto reversed = j | std::views::reverse;
+                CHECK(reversed == j_expected);
+            }
+
+            SECTION("transform")
+            {
+                json j
+                {
+                    { "a_key", "a_value"},
+                    { "b_key", "b_value"},
+                    { "c_key", "c_value"},
+                };
+                json j_expected{"a_key", "b_key", "c_key"};
+
+                // NOLINTNEXTLINE(fuchsia-trailing-return)
+                auto transformed = j.items() | std::views::transform([](const auto & item) -> std::string_view
+                {
+                    return item.key();
+                });
+                auto j_transformed = json::array();
+                std::ranges::copy(transformed, std::back_inserter(j_transformed));
+
+                CHECK(j_transformed == j_expected);
+            }
+        }
+    }
+#endif
+}
+
+#endif

tests/src/unit-iterators2.cpp

@@ -6,23 +6,17 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// When using macros that are only defined for particular versions of the standard
-// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
-// version macro in a comment close by, like this:
-// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. To avoid rebuilding this
+// large file for every standard, tests that depend on the standard version (e.g.,
+// those using JSON_HAS_FILESYSTEM, JSON_HAS_RANGES, or JSON_HAS_THREE_WAY_COMPARISON)
+// go into a separate file unit-iterators2-cpp<NN>.cpp. This file stays C++11-only.
 
 #include "doctest_compatibility.h"
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
-#if JSON_HAS_RANGES
-    #include <algorithm>
-    #include <ranges>
-#endif
-
 TEST_CASE("iterators 2")
 {
     SECTION("iterator comparisons")
@@ -872,94 +866,4 @@ TEST_CASE("iterators 2")
             }
         }
     }
-
-#if JSON_HAS_RANGES
-    // JSON_HAS_CPP_20 (do not remove; see note at top of file)
-    SECTION("ranges")
-    {
-        SECTION("concepts")
-        {
-            using nlohmann::detail::iteration_proxy_value;
-            CHECK(std::bidirectional_iterator<json::iterator>);
-            CHECK(std::input_iterator<iteration_proxy_value<json::iterator>>);
-
-            CHECK(std::is_same<json::iterator, std::ranges::iterator_t<json>>::value);
-            CHECK(std::ranges::bidirectional_range<json>);
-
-            using nlohmann::detail::iteration_proxy;
-            using items_type = decltype(std::declval<json&>().items());
-            CHECK(std::is_same<items_type, iteration_proxy<json::iterator>>::value);
-            CHECK(std::is_same<iteration_proxy_value<json::iterator>, std::ranges::iterator_t<items_type>>::value);
-            CHECK(std::ranges::input_range<items_type>);
-        }
-
-        SECTION("algorithms")
-        {
-            SECTION("copy")
-            {
-                json j{"foo", "bar"};
-                auto j_copied = json::array();
-
-                std::ranges::copy(j, std::back_inserter(j_copied));
-
-                CHECK(j == j_copied);
-            }
-
-            SECTION("find_if")
-            {
-                json j{1, 3, 2, 4};
-                auto j_even = json::array();
-
-#if JSON_USE_IMPLICIT_CONVERSIONS
-                auto it = std::ranges::find_if(j, [](int v) noexcept
-                {
-                    return (v % 2) == 0;
-                });
-#else
-                auto it = std::ranges::find_if(j, [](const json & j) noexcept
-                {
-                    int v;
-                    j.get_to(v);
-                    return (v % 2) == 0;
-                });
-#endif
-
-                CHECK(*it == 2);
-            }
-        }
-
-        SECTION("views")
-        {
-            SECTION("reverse")
-            {
-                json j{1, 2, 3, 4, 5};
-                json j_expected{5, 4, 3, 2, 1};
-
-                auto reversed = j | std::views::reverse;
-                CHECK(reversed == j_expected);
-            }
-
-            SECTION("transform")
-            {
-                json j
-                {
-                    { "a_key", "a_value"},
-                    { "b_key", "b_value"},
-                    { "c_key", "c_value"},
-                };
-                json j_expected{"a_key", "b_key", "c_key"};
-
-                // NOLINTNEXTLINE(fuchsia-trailing-return)
-                auto transformed = j.items() | std::views::transform([](const auto & item) -> std::string_view
-                {
-                    return item.key();
-                });
-                auto j_transformed = json::array();
-                std::ranges::copy(transformed, std::back_inserter(j_transformed));
-
-                CHECK(j_transformed == j_expected);
-            }
-        }
-    }
-#endif
 }

tests/src/unit-json_pointer-cpp20.cpp

@@ -0,0 +1,147 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++20-only part of unit-json_pointer.cpp (JSON pointer comparison
+// with C++20 rewritten candidates and operator<=>, and char8_t literals). It is kept in a
+// separate translation unit so the (much larger) unit-json_pointer.cpp is built for C++11
+// only and not rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+// capture whether JSON_DELETE_DEPRECATED_FUNCTIONS was enabled on the command
+// line *before* including json.hpp, since the library #undefs it once the header
+// has been fully processed (see include/nlohmann/detail/macro_unscope.hpp); the
+// tests of deprecated functions are skipped if these functions are deleted
+#if defined(JSON_DELETE_DEPRECATED_FUNCTIONS) && (JSON_DELETE_DEPRECATED_FUNCTIONS == 1)
+    #define JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
+#endif
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
+
+#ifdef JSON_HAS_CPP_20
+#if JSON_HAS_THREE_WAY_COMPARISON
+    #include <compare>
+#endif
+#include <string>
+#include <type_traits>
+
+TEST_CASE("JSON pointers (C++20)")
+{
+    SECTION("equality comparison")
+    {
+        std::string ptr_string{"/foo/bar"};
+        auto ptr1 = json::json_pointer(ptr_string);
+        auto ptr2 = json::json_pointer(ptr_string);
+
+        CHECK(ptr1 == ptr2);
+
+        CHECK_FALSE(ptr1 != ptr2);
+
+#ifndef JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
+        const char* ptr_cpstring = "/foo/bar";
+        const char ptr_castring[] = "/foo/bar"; // NOLINT(misc-const-correctness,hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+
+        CHECK(ptr1 == "/foo/bar");
+        CHECK(ptr1 == ptr_cpstring);
+        CHECK(ptr1 == ptr_castring);
+        CHECK(ptr1 == ptr_string);
+
+        CHECK("/foo/bar" == ptr1);
+        CHECK(ptr_cpstring == ptr1);
+        CHECK(ptr_castring == ptr1);
+        CHECK(ptr_string == ptr1);
+
+        CHECK_FALSE(ptr1 != "/foo/bar");
+        CHECK_FALSE(ptr1 != ptr_cpstring);
+        CHECK_FALSE(ptr1 != ptr_castring);
+        CHECK_FALSE(ptr1 != ptr_string);
+
+        CHECK_FALSE("/foo/bar" != ptr1);
+        CHECK_FALSE(ptr_cpstring != ptr1);
+        CHECK_FALSE(ptr_castring != ptr1);
+        CHECK_FALSE(ptr_string != ptr1);
+
+        SECTION("exceptions")
+        {
+            CHECK_THROWS_WITH_AS(ptr1 == "foo",
+                                 "[json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'foo'", json::parse_error&);
+            CHECK_THROWS_WITH_AS("foo" == ptr1,
+                                 "[json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'foo'", json::parse_error&);
+            CHECK_THROWS_WITH_AS(ptr1 == "/~~",
+                                 "[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'", json::parse_error&);
+            CHECK_THROWS_WITH_AS("/~~" == ptr1,
+                                 "[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'", json::parse_error&);
+        }
+#endif
+    }
+
+    SECTION("less-than comparison")
+    {
+        auto ptr1 = json::json_pointer("/foo/a");
+        auto ptr2 = json::json_pointer("/foo/b");
+
+#if JSON_HAS_THREE_WAY_COMPARISON
+        CHECK((ptr1 <=> ptr2) == std::strong_ordering::less); // *NOPAD*
+        CHECK(ptr2 > ptr1);
+#endif
+    }
+
+    SECTION("backwards compatibility and mixing")
+    {
+        json j = R"(
+        {
+            "foo": ["bar", "baz"]
+        }
+        )"_json;
+
+        using nlohmann::ordered_json;
+        using json_ptr_str = nlohmann::json_pointer<std::string>;
+        using json_ptr_j = nlohmann::json_pointer<json>;
+        using json_ptr_oj = nlohmann::json_pointer<ordered_json>;
+
+        std::string const ptr_string{"/foo/0"};
+        json_ptr_str ptr{ptr_string};
+        json_ptr_j ptr_j{ptr_string};
+        json_ptr_oj ptr_oj{ptr_string};
+
+        SECTION("equality comparison")
+        {
+            CHECK(ptr == ptr_j);
+            CHECK(ptr == ptr_oj);
+            CHECK(ptr_j == ptr);
+            CHECK(ptr_j == ptr_oj);
+            CHECK(ptr_oj == ptr_j);
+            CHECK(ptr_oj == ptr);
+
+            CHECK_FALSE(ptr != ptr_j);
+            CHECK_FALSE(ptr != ptr_oj);
+            CHECK_FALSE(ptr_j != ptr);
+            CHECK_FALSE(ptr_j != ptr_oj);
+            CHECK_FALSE(ptr_oj != ptr_j);
+            CHECK_FALSE(ptr_oj != ptr);
+        }
+    }
+
+#if defined(__cpp_char8_t)
+    SECTION("Using _json_pointer with char8_t literals #4945")
+    {
+        const json j = R"({"a": {"b": {"c": 123}}})"_json;
+        const auto p1 = "/a/b/c"_json_pointer;
+        CHECK(j[p1] == 123);
+
+        const auto p2 = u8"/a/b/c"_json_pointer;
+        CHECK(j[p2] == 123);
+    }
+#endif
+}
+
+#endif

tests/src/unit-json_pointer.cpp

@@ -779,14 +779,13 @@ TEST_CASE("JSON pointers")
         CHECK(ptr.to_string() == "/object/~1");
     }
 
+#if !JSON_HAS_THREE_WAY_COMPARISON // with three-way comparison, see unit-json_pointer-cpp20.cpp
     SECTION("equality comparison")
     {
         std::string ptr_string{"/foo/bar"};
         auto ptr1 = json::json_pointer(ptr_string);
         auto ptr2 = json::json_pointer(ptr_string);
 
-        // build with C++20 to test rewritten candidates
-        // JSON_HAS_CPP_20
 
         CHECK(ptr1 == ptr2);
 
@@ -829,6 +828,7 @@ TEST_CASE("JSON pointers")
         }
 #endif
     }
+#endif
 
     SECTION("less-than comparison")
     {
@@ -838,12 +838,6 @@ TEST_CASE("JSON pointers")
         CHECK(ptr1 < ptr2);
         CHECK_FALSE(ptr2 < ptr1);
 
-        // build with C++20
-        // JSON_HAS_CPP_20
-#if JSON_HAS_THREE_WAY_COMPARISON
-        CHECK((ptr1 <=> ptr2) == std::strong_ordering::less); // *NOPAD*
-        CHECK(ptr2 > ptr1);
-#endif
     }
 
     SECTION("usable as map key")
@@ -899,10 +893,9 @@ TEST_CASE("JSON pointers")
         CHECK_FALSE(ptr != ptr_j);
         CHECK_FALSE(ptr != ptr_oj);
 
+#if !JSON_HAS_THREE_WAY_COMPARISON // with three-way comparison, see unit-json_pointer-cpp20.cpp
         SECTION("equality comparison")
         {
-            // build with C++20 to test rewritten candidates
-            // JSON_HAS_CPP_20
 
             CHECK(ptr == ptr_j);
             CHECK(ptr == ptr_oj);
@@ -918,6 +911,7 @@ TEST_CASE("JSON pointers")
             CHECK_FALSE(ptr_oj != ptr_j);
             CHECK_FALSE(ptr_oj != ptr);
         }
+#endif
     }
 
     SECTION("value(json_pointer, default) with ordered_json #5664")
@@ -939,19 +933,6 @@ TEST_CASE("JSON pointers")
         CHECK(j.value(ptr_missing, 42) == 42);
     }
 
-    // build with C++20
-    // JSON_HAS_CPP_20
-#if defined(__cpp_char8_t)
-    SECTION("Using _json_pointer with char8_t literals #4945")
-    {
-        const json j = R"({"a": {"b": {"c": 123}}})"_json;
-        const auto p1 = "/a/b/c"_json_pointer;
-        CHECK(j[p1] == 123);
-
-        const auto p2 = u8"/a/b/c"_json_pointer;
-        CHECK(j[p2] == 123);
-    }
-#endif
 }
 
 TEST_CASE("unescaping keeps a '~' that does not start an escape sequence")

tests/src/unit-json_view_builder.cpp

@@ -173,7 +173,7 @@ void check_string(const std::string& content)
 }
 
 // a small deterministic generator of documents
-struct generator
+struct builder_generator
 {
     std::mt19937 rng{5295}; // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed)
 
@@ -407,7 +407,7 @@ TEST_CASE("json_view builder")
 
     SECTION("generated documents and damaged copies")
     {
-        generator g;
+        builder_generator g;
         for (int i = 0; i < 3000; ++i)
         {
             std::string text;

tests/src/unit-json_view_dump.cpp

@@ -94,6 +94,58 @@ TEST_CASE("json_view dump")
         CHECK(d.root().dump() == json::parse(text).dump());
         CHECK(d.root().dump() == "[1.5,100.0,0,-0.0,1.2345678901234568e+29,18446744073709551615,-9223372036854775808,0.1,1e-07,5e-324]");
         CHECK(d.root().dump(-1, ' ', false, json_view::number_format::source) == "[1.50,1E2,-0,-0.0,123456789012345678901234567890,18446744073709551615,-9223372036854775808,0.1,1e-7,5e-324]");
+        // also indented, and with ensure_ascii
+        CHECK(d.root().dump(0, ' ', false, json_view::number_format::source) == "[\n1.50,\n1E2,\n-0,\n-0.0,\n123456789012345678901234567890,\n18446744073709551615,\n-9223372036854775808,\n0.1,\n1e-7,\n5e-324\n]");
+        CHECK(d.root().dump(-1, ' ', true, json_view::number_format::source) == "[1.50,1E2,-0,-0.0,123456789012345678901234567890,18446744073709551615,-9223372036854775808,0.1,1e-7,5e-324]");
+
+        // float tokens of up to 17 significant digits in every spelling: those
+        // of at most 15 digits are written from their digits, the others
+        // through the conversion; both as dump() writes them
+        {
+            std::mt19937_64 tokens(1170); // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed)
+            // a number below n; the remainder is a std::uint64_t, which is
+            // std::size_t on some platforms and wider on others
+            const auto draw = [&tokens](std::size_t n)
+            {
+                const std::uint64_t r = tokens() % n;
+                return static_cast<std::size_t>(r);
+            };
+            std::string many_tokens = "[";
+            for (int i = 0; i < 20000; ++i)
+            {
+                const std::size_t length = 1 + draw(17);
+                std::string digits(1, static_cast<char>('1' + draw(9)));
+                for (std::size_t k = 1; k < length; ++k)
+                {
+                    digits += static_cast<char>('0' + draw(10));
+                }
+                digits += std::string(draw(4), '0'); // trailing zeros
+                std::string token = draw(3) == 0 ? "-" : "";
+                const std::size_t point = draw(digits.size() + 1);
+                if (point == 0)
+                {
+                    token += "0." + std::string(draw(5), '0') + digits;
+                }
+                else
+                {
+                    token += digits.substr(0, point) + (point < digits.size() ? "." + digits.substr(point) : "");
+                }
+                // an exponent that keeps the value between about 1e-320 and 1e300
+                const int exponent = static_cast<int>(draw(600)) - 300 - static_cast<int>(point);
+                if (draw(4) != 0)
+                {
+                    token += (draw(2) == 0 ? "e" : "E") + std::string(exponent >= 0 && draw(2) == 0 ? "+" : "") + std::to_string(exponent);
+                }
+                else if (point == digits.size())
+                {
+                    token += ".0"; // (a float, not an integer)
+                }
+                many_tokens += (i != 0 ? "," : "") + token;
+            }
+            many_tokens += ']';
+            const json_document many_doc = json_document::parse(many_tokens);
+            CHECK(many_doc.root().dump() == json::parse(many_tokens).dump());
+        }
 
         // random doubles, written as parse() and dump() would
         std::mt19937_64 rng(1170); // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed)

tests/src/unit-json_view_edit.cpp

@@ -19,13 +19,18 @@ using nlohmann::ordered_json_editable_document;
 using nlohmann::ordered_json_editable_view;
 using ptr_t = ordered_json::json_pointer;
 
+#include "json_view_test_helpers.hpp"
+#if !defined(JSON_NOEXCEPTION)
+    using json_view_test::exception_of_call;
+#endif
+
 #include <array>
 #include <cmath>
 #include <cstring>
 #include <cstdint>
-#include <functional>
 #include <iterator>
 #include <limits>
+#include <map>
 #include <memory>
 #include <random>
 #include <set>
@@ -34,7 +39,7 @@ using ptr_t = ordered_json::json_pointer;
 
 namespace
 {
-std::uint32_t rng()
+std::uint32_t edit_rng()
 {
     static std::mt19937 generator(5295); // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed): reproducible
     // result_type is std::uint_fast32_t, which may be wider than 32 bits
@@ -44,12 +49,12 @@ std::uint32_t rng()
 
 int r(int n)
 {
-    return static_cast<int>(rng() % static_cast<unsigned>(n));
+    return static_cast<int>(edit_rng() % static_cast<unsigned>(n));
 }
 
-int counter = 0;
+int edit_counter = 0;
 
-std::uint64_t bits(double x)
+std::uint64_t edit_bits(double x)
 {
     std::uint64_t b = 0;
     std::memcpy(&b, &x, sizeof(b));
@@ -76,11 +81,11 @@ ordered_json random_scalar()
         case 1:
             return r(2) == 0;
         case 2:
-            return static_cast<std::int64_t>(rng()) - 2147483648LL;
+            return static_cast<std::int64_t>(edit_rng()) - 2147483648LL;
         case 3:
-            return (static_cast<std::uint64_t>(rng()) * 4294967296ULL) + rng();
+            return (static_cast<std::uint64_t>(edit_rng()) * 4294967296ULL) + edit_rng();
         case 4:
-            return static_cast<double>(static_cast<std::int32_t>(rng())) / (1 + r(1000));
+            return static_cast<double>(static_cast<std::int32_t>(edit_rng())) / (1 + r(1000));
         case 5:
             return r(2) == 0 ? 1e300 * (r(2) == 0 ? 1 : -1) : 5e-324;
         case 6:
@@ -98,7 +103,7 @@ ordered_json random_value(int depth)
         ordered_json o = ordered_json::object();
         for (int i = r(5); i > 0; --i)
         {
-            o[random_string() + "#" + std::to_string(counter++)] = random_value(depth + 1);
+            o[random_string() + "#" + std::to_string(edit_counter++)] = random_value(depth + 1);
         }
         return o;
     }
@@ -173,7 +178,7 @@ void compare(const ordered_json_editable_view& v, const ordered_json& j)
     }
     else if (j.is_number_float())
     {
-        CHECK(bits(v.get<double>()) == bits(j.get<double>()));
+        CHECK(edit_bits(v.get<double>()) == edit_bits(j.get<double>()));
     }
     else if (j.is_number_integer())
     {
@@ -430,7 +435,7 @@ TEST_CASE("json_view edits: differential")
                 }
                 else if ((op == 4 || op == 5) && target.is_object()) // set a member (new or existing)
                 {
-                    std::string key = random_string() + "#" + std::to_string(counter++);
+                    std::string key = random_string() + "#" + std::to_string(edit_counter++);
                     if (op == 5 && !target.empty())
                     {
                         key = std::next(target.begin(), r(static_cast<int>(target.size()))).key();
@@ -512,25 +517,6 @@ TEST_CASE("json_view edits: differential")
     }
 }
 
-namespace
-{
-#if !defined(JSON_NOEXCEPTION)
-// the exception a call throws, or "" if it throws none
-std::string exception_of_call(const std::function<void()>& f)
-{
-    try
-    {
-        f();
-    }
-    catch (const json::exception& e)
-    {
-        return e.what();
-    }
-    return "";
-}
-#endif
-} // namespace
-
 TEST_CASE("json_view edits: errors")
 {
     SECTION("an empty document")
@@ -586,316 +572,7 @@ TEST_CASE("json_view edits: errors")
     static_cast<void>(other);
 }
 
-TEST_CASE("json_view edits: views and values")
-{
-    SECTION("a value that is no longer part of the document")
-    {
-        json_editable_document d = json_editable_document::parse("[[[1,2]]]");
-        const json_editable_view inner = d.root()[0][0];
-        d.set(d.root()[0], json::array({7}));
-        d.set(inner, 5);
-        CHECK(d.root().dump() == "[[7]]");
-        CHECK(inner.get<int>() == 5);
-    }
-
-    SECTION("views keep referring to their value")
-    {
-        json_editable_document d = json_editable_document::parse(R"({"a": [10, 20, 30], "b": {"c": "text"}})");
-        const json_editable_view a = d.root()["a"];
-        const json_editable_view twenty = a[1];
-        const json_editable_view c = d.root()["b"]["c"];
-        d.insert(a, 0, 5);
-        d.push_back(a, 40);
-        CHECK(twenty.get<int>() == 20);
-        CHECK(a[2].get<int>() == 20);
-        d.erase(a, 2);
-        CHECK(twenty.get<int>() == 20); // an erased value keeps its last value
-        d.set(c, 7);
-        CHECK(c.get<int>() == 7); // a held view sees an assignment
-        d.set(d.root()["b"], json::array({1, 2}));
-        CHECK(d.root()["b"].dump() == "[1,2]");
-        CHECK(d.root().dump() == R"({"a":[5,10,30,40],"b":[1,2]})");
-        CHECK(d.root()["a"][0].source_offset() == static_cast<std::size_t>(-1)); // a new value
-        CHECK(d.root()["a"][1].source_offset() != static_cast<std::size_t>(-1));
-    }
-
-    SECTION("strings stay valid while more edits come")
-    {
-        json_editable_document d = json_editable_document::parse("[]");
-        const auto first = d.push_back(d.root(), std::string(100, 'x')).get_string();
-        for (int i = 0; i < 1000; ++i)
-        {
-            d.push_back(d.root(), std::string(static_cast<std::size_t>(i % 50), 'y'));
-        }
-        CHECK(std::string(first.data(), first.size()) == std::string(100, 'x'));
-        CHECK(d.root().size() == 1001);
-    }
-
-    SECTION("numbers")
-    {
-        json_editable_document d = json_editable_document::parse(R"([1.50, 1E2, 3])");
-        d.set(d.root(), 2, 0.1);
-        d.push_back(d.root(), std::numeric_limits<double>::quiet_NaN());
-        d.push_back(d.root(), -std::numeric_limits<double>::infinity());
-        d.push_back(d.root(), (std::numeric_limits<std::uint64_t>::max)());
-        d.push_back(d.root(), (std::numeric_limits<std::int64_t>::min)());
-        CHECK(d.root().dump() == "[1.5,100.0,0.1,null,null,18446744073709551615,-9223372036854775808]");
-        CHECK(d.root().dump(-1, ' ', false, json_editable_view::number_format::source) == "[1.50,1E2,0.1,null,null,18446744073709551615,-9223372036854775808]");
-        CHECK(std::isnan(d.root()[3].get<double>()));
-        CHECK(std::isinf(d.root()[4].get<double>()));
-        CHECK(d.root()[2].number_token() == "0.1");
-        CHECK(d.root()[5].get<std::uint64_t>() == 18446744073709551615u);
-        CHECK(d.root()[6].number_token() == "-9223372036854775808");
-        CHECK(d.root().materialize().dump() == json::parse(R"([1.5, 100.0, 0.1, null, null, 18446744073709551615, -9223372036854775808])").dump());
-    }
-
-    SECTION("nulls become containers, and the root can be replaced")
-    {
-        json_editable_document d = json_editable_document::parse("[null, null]");
-        d.set(d.root()[0], "k", 1);
-        d.push_back(d.root()[1], true);
-        CHECK(d.root().dump() == R"([{"k":1},[true]])");
-        d.set(d.root(), "scalar");
-        CHECK(d.root().dump() == R"("scalar")");
-        d.set(d.root(), json{{"x", {1, 2}}});
-        d.set(json::json_pointer("/x/-"), 3);
-        d.set(json::json_pointer("/x/3"), 4); // the size of the array appends too
-        d.set(json::json_pointer("/y"), false);
-        CHECK(d.root().dump() == R"({"x":[1,2,3,4],"y":false})");
-        CHECK(d.erase(json::json_pointer("/x/0")) == 1);
-        CHECK(d.erase(json::json_pointer("/y")) == 1);
-        CHECK(d.erase(json::json_pointer("/nothing")) == 0);
-        CHECK(d.root().dump() == R"({"x":[2,3,4]})");
-    }
-
-    SECTION("duplicate keys")
-    {
-        json_editable_document d = json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3})");
-        d.set(d.root(), "a", 4); // the first member is assigned, the others dropped
-        CHECK(d.root().dump() == R"({"a":4,"b":2})");
-        d = json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3})");
-        CHECK(d.erase(d.root(), "a") == 2);
-        CHECK(d.root().dump() == R"({"b":2})");
-    }
-
-    SECTION("values from other documents")
-    {
-        const json_document source = json_document::parse(R"({"list": [1, "two", {"three": 3.5}], "text": "a\nb"})");
-        json_editable_document edited = json_editable_document::parse("[0]");
-        edited.set(edited.root(), 0, json{{"inner", {1, 2}}});
-        json_editable_document d = json_editable_document::parse("{}");
-        d.set(d.root(), "copy", source.root()["list"]);
-        d.set(d.root(), "text", source.root()["text"]);
-        d.set(d.root(), "edited", edited.root()[0]);
-        d.set(d.root(), "self", d.root()["copy"]);
-        // (a raw string with a backslash must not be a macro argument: MSVC C2017)
-        const std::string expected = R"({"copy":[1,"two",{"three":3.5}],"text":"a\nb","edited":{"inner":[1,2]},"self":[1,"two",{"three":3.5}]})";
-        CHECK(d.root().dump() == expected);
-        CHECK(d.root()["copy"] == source.root()["list"]);
-        CHECK(source.root()["list"] == d.root()["self"]);
-        CHECK(d.root() != source.root());
-    }
-
-    SECTION("large objects")
-    {
-        std::string text = "{";
-        for (int i = 0; i < 200; ++i)
-        {
-            text += (i != 0 ? ",\"k" : "\"k") + std::to_string(i) + "\":" + std::to_string(i);
-        }
-        text += '}';
-        json_editable_document d = json_editable_document::parse(text);
-        d.set(d.root(), "k7", "seven"); // assigned in place: the index stays in use
-        CHECK(d.root()["k7"].get_string() == "seven");
-        d.set(d.root(), "new", 1); // appended: the members move, the lookup is linear
-        CHECK(d.root()["new"].get<int>() == 1);
-        CHECK(d.root()["k199"].get<int>() == 199);
-        d.erase(d.root(), "k0");
-        CHECK(!d.root().contains("k0"));
-        CHECK(d.root().size() == 200);
-    }
-
-    SECTION("reuse and memory")
-    {
-        json_editable_document d = json_editable_document::parse("[1, 2, 3]");
-        const std::size_t before = d.memory_usage();
-        for (int i = 0; i < 100; ++i)
-        {
-            d.push_back(d.root(), "some text");
-        }
-        CHECK(d.memory_usage() > before);
-        const json_editable_view first = d.root()[0];
-        d.shrink_to_fit(); // (with edits, the index stays in place)
-        CHECK(first.get<int>() == 1);
-        d.read(std::string("[true]"));
-        CHECK(d.root().dump() == "[true]");
-        d.push_back(d.root(), false);
-        CHECK(d.root().dump() == "[true,false]");
-    }
-}
-
-TEST_CASE("json_view edits: deeply nested values")
-{
-    // copying a value into a document must not recurse per nesting level
-    const std::size_t depth = 100000;
-    const std::string brackets = std::string(depth, '[') + std::string(depth, ']');
-    std::string braces;
-    for (std::size_t i = 0; i < depth; ++i)
-    {
-        braces += "{\"a\":";
-    }
-    braces += '1';
-    braces += std::string(depth, '}');
-
-    SECTION("a view of a read-only document")
-    {
-        const json_document source = json_document::parse(brackets);
-        json_editable_document d = json_editable_document::parse("[]");
-        d.push_back(d.root(), source.root());
-        CHECK(d.root().dump() == "[" + brackets + "]");
-    }
-
-    SECTION("a view of an editable document")
-    {
-        const json_editable_document source = json_editable_document::parse(braces);
-        json_editable_document d = json_editable_document::parse("{}");
-        d.set(d.root(), "deep", source.root());
-        CHECK(d.root().dump() == "{\"deep\":" + braces + "}");
-    }
-
-    SECTION("a view of an edited document (values behind links)")
-    {
-        const json_document source = json_document::parse(brackets);
-        json_editable_document edited = json_editable_document::parse("[[]]");
-        edited.push_back(edited.root()[0], source.root());
-        edited.push_back(edited.root(), source.root());
-        json_editable_document d = json_editable_document::parse("null");
-        d.set(d.root(), edited.root());
-        CHECK(d.root().dump() == "[[" + brackets + "]," + brackets + "]");
-    }
-
-    SECTION("a basic_json value")
-    {
-        json deep = json::array();
-        json* inner = &deep;
-        for (std::size_t i = 1; i < depth; ++i)
-        {
-            inner->push_back(json::array());
-            inner = &inner->back();
-        }
-        json_editable_document d = json_editable_document::parse("[]");
-        d.push_back(d.root(), deep);
-        CHECK(d.root().dump() == "[" + brackets + "]");
-    }
-
-    SECTION("a basic_json value with objects")
-    {
-        json deep = 1;
-        for (std::size_t i = 0; i < depth; ++i)
-        {
-            json outer = json::object();
-            outer["a"] = std::move(deep);
-            deep = std::move(outer);
-        }
-        json_editable_document d = json_editable_document::parse("{}");
-        d.set(d.root(), "deep", deep);
-        CHECK(d.root().dump() == "{\"deep\":" + braces + "}");
-    }
-}
-
 #if !defined(JSON_NOEXCEPTION)
-TEST_CASE("json_view edits: pointers below a null value")
-{
-    // a null value on the way becomes what basic_json makes of it: an array
-    // for "-" and for digits, an object otherwise
-    struct test_case
-    {
-        const char* document;
-        const char* pointer;
-    };
-    const std::array<test_case, 16> cases =
-    {
-        {
-            {R"({"a":null})", "/a/0"},
-            {R"({"a":null})", "/a/-"},
-            {R"({"a":null})", "/a/3"},
-            {R"({"a":null})", "/a/x"},
-            {R"({"a":null})", "/a/+1"},
-            {R"({"a":null})", "/a/01"},
-            {R"({"a":null})", "/a/"},
-            {R"({"a":{"b":null}})", "/a/b/1"},
-            {R"({"a":{"b":null}})", "/a/b/-"},
-            {R"({"a":[null]})", "/a/0/0"},
-            {R"({"a":[null,null]})", "/a/1/k"},
-            {R"([null])", "/0"},
-            {"null", "/0"},
-            {"null", "/-"},
-            {"null", "/k"},
-            {"null", ""},
-        }
-    };
-    for (const test_case& c : cases)
-    {
-        CAPTURE(c.document)
-        CAPTURE(c.pointer)
-        json expected = json::parse(c.document);
-        const std::string error = exception_of_call([&]
-        {
-            expected[json::json_pointer(c.pointer)] = 1;
-        });
-        json_editable_document d = json_editable_document::parse(c.document);
-        if (error.empty())
-        {
-            d.set(json::json_pointer(c.pointer), 1);
-            CHECK(d.root().dump() == expected.dump());
-            CHECK(d.root().materialize() == expected);
-        }
-        else
-        {
-            // the same error, and the document is not changed
-            CHECK(exception_of_call([&] { d.set(json::json_pointer(c.pointer), 1); }) == error);
-            CHECK(d.root().dump() == json::parse(c.document).dump());
-        }
-    }
-}
-
-TEST_CASE("json_view edits: strings of other documents are checked")
-{
-    // A document borrows the text it was parsed from, and sees later changes
-    // of the text: a way to get ill-formed UTF-8 into a view. Copying it into
-    // an editable document is an error, as for any other string.
-    std::string text = R"({"key":"abc","list":["abc"]})";
-    const json_document source = json_document::parse(text);
-    const auto message_of = [](const std::string & bad)
-    {
-        return exception_of_call([&]
-        {
-            const std::string dumped = json(bad).dump();
-            static_cast<void>(dumped);
-        });
-    };
-    json_editable_document d = json_editable_document::parse("[1]");
-    d.push_back(d.root(), source.root());
-    CHECK(d.root().dump() == R"([1,{"key":"abc","list":["abc"]}])");
-
-    text[text.find("abc") + 1] = '\xC3'; // "a\xC3c"
-    text[text.rfind("abc") + 1] = '\xC3';
-    const std::string bad_value = message_of(std::string("a\xC3" "c"));
-    CHECK(!bad_value.empty());
-    CHECK(exception_of_call([&] { d.push_back(d.root(), source.root()["key"]); }) == bad_value);
-    CHECK(exception_of_call([&] { d.set(d.root()[0], source.root()["list"][0]); }) == bad_value);
-    CHECK(exception_of_call([&] { d.set(d.root(), 0, source.root()["list"]); }) == bad_value);
-    CHECK(exception_of_call([&] { d.set(d.root(), 0, source.root()); }) == bad_value);
-
-    text[text.find("key") + 1] = '\xC3'; // a key is checked as well
-    const std::string bad_key = message_of(std::string("k\xC3" "y"));
-    CHECK(exception_of_call([&] { d.set(d.root(), 0, source.root()); }) == bad_key);
-    CHECK(exception_of_call([&] { d.push_back(d.root(), source.root()); }) == bad_key);
-
-    // nothing of the failed edits is visible
-    CHECK(d.root().dump() == R"([1,{"key":"abc","list":["abc"]}])");
-}
-
 TEST_CASE("json_view edits: the size of a text arena")
 {
     using nlohmann::detail::view::text_capacity;

tests/src/unit-json_view_edit_values.cpp

@@ -0,0 +1,357 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json_view.hpp>
+using nlohmann::json;
+using nlohmann::json_document;
+using nlohmann::json_editable_document;
+using nlohmann::json_editable_view;
+
+#include "json_view_test_helpers.hpp"
+#if !defined(JSON_NOEXCEPTION)
+    using json_view_test::exception_of_call;
+#endif
+
+#include <array>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <map>
+#include <string>
+#include <utility>
+#include <vector>
+
+// These tests were split off unit-json_view_edit.cpp, whose object file got too
+// large for the MinGW linker (see json_view_test_helpers.hpp).
+
+TEST_CASE("json_view edits: views and values")
+{
+    SECTION("a value that is no longer part of the document")
+    {
+        json_editable_document d = json_editable_document::parse("[[[1,2]]]");
+        const json_editable_view inner = d.root()[0][0];
+        d.set(d.root()[0], json::array({7}));
+        d.set(inner, 5);
+        CHECK(d.root().dump() == "[[7]]");
+        CHECK(inner.get<int>() == 5);
+    }
+
+    SECTION("views keep referring to their value")
+    {
+        json_editable_document d = json_editable_document::parse(R"({"a": [10, 20, 30], "b": {"c": "text"}})");
+        const json_editable_view a = d.root()["a"];
+        const json_editable_view twenty = a[1];
+        const json_editable_view c = d.root()["b"]["c"];
+        d.insert(a, 0, 5);
+        d.push_back(a, 40);
+        CHECK(twenty.get<int>() == 20);
+        CHECK(a[2].get<int>() == 20);
+        d.erase(a, 2);
+        CHECK(twenty.get<int>() == 20); // an erased value keeps its last value
+        d.set(c, 7);
+        CHECK(c.get<int>() == 7); // a held view sees an assignment
+        d.set(d.root()["b"], json::array({1, 2}));
+        CHECK(d.root()["b"].dump() == "[1,2]");
+        CHECK(d.root().dump() == R"({"a":[5,10,30,40],"b":[1,2]})");
+        CHECK(d.root()["a"][0].source_offset() == static_cast<std::size_t>(-1)); // a new value
+        CHECK(d.root()["a"][1].source_offset() != static_cast<std::size_t>(-1));
+    }
+
+    SECTION("strings stay valid while more edits come")
+    {
+        json_editable_document d = json_editable_document::parse("[]");
+        const auto first = d.push_back(d.root(), std::string(100, 'x')).get_string();
+        for (int i = 0; i < 1000; ++i)
+        {
+            d.push_back(d.root(), std::string(static_cast<std::size_t>(i % 50), 'y'));
+        }
+        CHECK(std::string(first.data(), first.size()) == std::string(100, 'x'));
+        CHECK(d.root().size() == 1001);
+    }
+
+    SECTION("numbers")
+    {
+        json_editable_document d = json_editable_document::parse(R"([1.50, 1E2, 3])");
+        d.set(d.root(), 2, 0.1);
+        d.push_back(d.root(), std::numeric_limits<double>::quiet_NaN());
+        d.push_back(d.root(), -std::numeric_limits<double>::infinity());
+        d.push_back(d.root(), (std::numeric_limits<std::uint64_t>::max)());
+        d.push_back(d.root(), (std::numeric_limits<std::int64_t>::min)());
+        CHECK(d.root().dump() == "[1.5,100.0,0.1,null,null,18446744073709551615,-9223372036854775808]");
+        CHECK(d.root().dump(-1, ' ', false, json_editable_view::number_format::source) == "[1.50,1E2,0.1,null,null,18446744073709551615,-9223372036854775808]");
+        CHECK(std::isnan(d.root()[3].get<double>()));
+        CHECK(std::isinf(d.root()[4].get<double>()));
+        CHECK(d.root()[2].number_token() == "0.1");
+        CHECK(d.root()[5].get<std::uint64_t>() == 18446744073709551615u);
+        CHECK(d.root()[6].number_token() == "-9223372036854775808");
+        CHECK(d.root().materialize().dump() == json::parse(R"([1.5, 100.0, 0.1, null, null, 18446744073709551615, -9223372036854775808])").dump());
+    }
+
+    SECTION("numbers of other float types")
+    {
+        // doubles have their own path to the output; other float types are
+        // written as basic_json writes them, non-finite values as null
+        using json_float = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
+        using document_float = nlohmann::basic_json_document<json_float, true>;
+        document_float d = document_float::parse("[1.5]");
+        d.push_back(d.root(), std::numeric_limits<float>::quiet_NaN());
+        d.push_back(d.root(), -std::numeric_limits<float>::infinity());
+        CHECK(d.root().dump() == "[1.5,null,null]");
+        CHECK(d.root().dump(2) == json_float::parse("[1.5, null, null]").dump(2));
+    }
+
+    SECTION("nulls become containers, and the root can be replaced")
+    {
+        json_editable_document d = json_editable_document::parse("[null, null]");
+        d.set(d.root()[0], "k", 1);
+        d.push_back(d.root()[1], true);
+        CHECK(d.root().dump() == R"([{"k":1},[true]])");
+        d.set(d.root(), "scalar");
+        CHECK(d.root().dump() == R"("scalar")");
+        d.set(d.root(), json{{"x", {1, 2}}});
+        d.set(json::json_pointer("/x/-"), 3);
+        d.set(json::json_pointer("/x/3"), 4); // the size of the array appends too
+        d.set(json::json_pointer("/y"), false);
+        CHECK(d.root().dump() == R"({"x":[1,2,3,4],"y":false})");
+        CHECK(d.erase(json::json_pointer("/x/0")) == 1);
+        CHECK(d.erase(json::json_pointer("/y")) == 1);
+        CHECK(d.erase(json::json_pointer("/nothing")) == 0);
+        CHECK(d.root().dump() == R"({"x":[2,3,4]})");
+    }
+
+    SECTION("duplicate keys")
+    {
+        json_editable_document d = json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3})");
+        d.set(d.root(), "a", 4); // the first member is assigned, the others dropped
+        CHECK(d.root().dump() == R"({"a":4,"b":2})");
+        d = json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3})");
+        CHECK(d.erase(d.root(), "a") == 2);
+        CHECK(d.root().dump() == R"({"b":2})");
+    }
+
+    SECTION("values from other documents")
+    {
+        const json_document source = json_document::parse(R"({"list": [1, "two", {"three": 3.5}], "text": "a\nb"})");
+        json_editable_document edited = json_editable_document::parse("[0]");
+        edited.set(edited.root(), 0, json{{"inner", {1, 2}}});
+        json_editable_document d = json_editable_document::parse("{}");
+        d.set(d.root(), "copy", source.root()["list"]);
+        d.set(d.root(), "text", source.root()["text"]);
+        d.set(d.root(), "edited", edited.root()[0]);
+        d.set(d.root(), "self", d.root()["copy"]);
+        // (a raw string with a backslash must not be a macro argument: MSVC C2017)
+        const std::string expected = R"({"copy":[1,"two",{"three":3.5}],"text":"a\nb","edited":{"inner":[1,2]},"self":[1,"two",{"three":3.5}]})";
+        CHECK(d.root().dump() == expected);
+        CHECK(d.root()["copy"] == source.root()["list"]);
+        CHECK(source.root()["list"] == d.root()["self"]);
+        CHECK(d.root() != source.root());
+    }
+
+    SECTION("large objects")
+    {
+        std::string text = "{";
+        for (int i = 0; i < 200; ++i)
+        {
+            text += (i != 0 ? ",\"k" : "\"k") + std::to_string(i) + "\":" + std::to_string(i);
+        }
+        text += '}';
+        json_editable_document d = json_editable_document::parse(text);
+        d.set(d.root(), "k7", "seven"); // assigned in place: the index stays in use
+        CHECK(d.root()["k7"].get_string() == "seven");
+        d.set(d.root(), "new", 1); // appended: the members move, the lookup is linear
+        CHECK(d.root()["new"].get<int>() == 1);
+        CHECK(d.root()["k199"].get<int>() == 199);
+        d.erase(d.root(), "k0");
+        CHECK(!d.root().contains("k0"));
+        CHECK(d.root().size() == 200);
+    }
+
+    SECTION("reuse and memory")
+    {
+        json_editable_document d = json_editable_document::parse("[1, 2, 3]");
+        const std::size_t before = d.memory_usage();
+        for (int i = 0; i < 100; ++i)
+        {
+            d.push_back(d.root(), "some text");
+        }
+        CHECK(d.memory_usage() > before);
+        const json_editable_view first = d.root()[0];
+        d.shrink_to_fit(); // (with edits, the index stays in place)
+        CHECK(first.get<int>() == 1);
+        d.read(std::string("[true]"));
+        CHECK(d.root().dump() == "[true]");
+        d.push_back(d.root(), false);
+        CHECK(d.root().dump() == "[true,false]");
+    }
+}
+
+TEST_CASE("json_view edits: deeply nested values")
+{
+    // copying a value into a document must not recurse per nesting level
+    const std::size_t depth = 100000;
+    const std::string brackets = std::string(depth, '[') + std::string(depth, ']');
+    std::string braces;
+    for (std::size_t i = 0; i < depth; ++i)
+    {
+        braces += "{\"a\":";
+    }
+    braces += '1';
+    braces += std::string(depth, '}');
+
+    SECTION("a view of a read-only document")
+    {
+        const json_document source = json_document::parse(brackets);
+        json_editable_document d = json_editable_document::parse("[]");
+        d.push_back(d.root(), source.root());
+        CHECK(d.root().dump() == "[" + brackets + "]");
+    }
+
+    SECTION("a view of an editable document")
+    {
+        const json_editable_document source = json_editable_document::parse(braces);
+        json_editable_document d = json_editable_document::parse("{}");
+        d.set(d.root(), "deep", source.root());
+        CHECK(d.root().dump() == "{\"deep\":" + braces + "}");
+    }
+
+    SECTION("a view of an edited document (values behind links)")
+    {
+        const json_document source = json_document::parse(brackets);
+        json_editable_document edited = json_editable_document::parse("[[]]");
+        edited.push_back(edited.root()[0], source.root());
+        edited.push_back(edited.root(), source.root());
+        json_editable_document d = json_editable_document::parse("null");
+        d.set(d.root(), edited.root());
+        CHECK(d.root().dump() == "[[" + brackets + "]," + brackets + "]");
+    }
+
+    SECTION("a basic_json value")
+    {
+        json deep = json::array();
+        json* inner = &deep;
+        for (std::size_t i = 1; i < depth; ++i)
+        {
+            inner->push_back(json::array());
+            inner = &inner->back();
+        }
+        json_editable_document d = json_editable_document::parse("[]");
+        d.push_back(d.root(), deep);
+        CHECK(d.root().dump() == "[" + brackets + "]");
+    }
+
+    SECTION("a basic_json value with objects")
+    {
+        json deep = 1;
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            json outer = json::object();
+            outer["a"] = std::move(deep);
+            deep = std::move(outer);
+        }
+        json_editable_document d = json_editable_document::parse("{}");
+        d.set(d.root(), "deep", deep);
+        CHECK(d.root().dump() == "{\"deep\":" + braces + "}");
+    }
+}
+
+#if !defined(JSON_NOEXCEPTION)
+TEST_CASE("json_view edits: pointers below a null value")
+{
+    // a null value on the way becomes what basic_json makes of it: an array
+    // for "-" and for digits, an object otherwise
+    struct test_case
+    {
+        const char* document;
+        const char* pointer;
+    };
+    const std::array<test_case, 16> cases =
+    {
+        {
+            {R"({"a":null})", "/a/0"},
+            {R"({"a":null})", "/a/-"},
+            {R"({"a":null})", "/a/3"},
+            {R"({"a":null})", "/a/x"},
+            {R"({"a":null})", "/a/+1"},
+            {R"({"a":null})", "/a/01"},
+            {R"({"a":null})", "/a/"},
+            {R"({"a":{"b":null}})", "/a/b/1"},
+            {R"({"a":{"b":null}})", "/a/b/-"},
+            {R"({"a":[null]})", "/a/0/0"},
+            {R"({"a":[null,null]})", "/a/1/k"},
+            {R"([null])", "/0"},
+            {"null", "/0"},
+            {"null", "/-"},
+            {"null", "/k"},
+            {"null", ""},
+        }
+    };
+    for (const test_case& c : cases)
+    {
+        CAPTURE(c.document)
+        CAPTURE(c.pointer)
+        json expected = json::parse(c.document);
+        const std::string error = exception_of_call([&]
+        {
+            expected[json::json_pointer(c.pointer)] = 1;
+        });
+        json_editable_document d = json_editable_document::parse(c.document);
+        if (error.empty())
+        {
+            d.set(json::json_pointer(c.pointer), 1);
+            CHECK(d.root().dump() == expected.dump());
+            CHECK(d.root().materialize() == expected);
+        }
+        else
+        {
+            // the same error, and the document is not changed
+            CHECK(exception_of_call([&] { d.set(json::json_pointer(c.pointer), 1); }) == error);
+            CHECK(d.root().dump() == json::parse(c.document).dump());
+        }
+    }
+}
+
+TEST_CASE("json_view edits: strings of other documents are checked")
+{
+    // A document borrows the text it was parsed from, and sees later changes
+    // of the text: a way to get ill-formed UTF-8 into a view. Copying it into
+    // an editable document is an error, as for any other string.
+    std::string text = R"({"key":"abc","list":["abc"]})";
+    const json_document source = json_document::parse(text);
+    const auto message_of = [](const std::string & bad)
+    {
+        return exception_of_call([&]
+        {
+            const std::string dumped = json(bad).dump();
+            static_cast<void>(dumped);
+        });
+    };
+    json_editable_document d = json_editable_document::parse("[1]");
+    d.push_back(d.root(), source.root());
+    CHECK(d.root().dump() == R"([1,{"key":"abc","list":["abc"]}])");
+
+    text[text.find("abc") + 1] = '\xC3'; // "a\xC3c"
+    text[text.rfind("abc") + 1] = '\xC3';
+    const std::string bad_value = message_of(std::string("a\xC3" "c"));
+    CHECK(!bad_value.empty());
+    CHECK(exception_of_call([&] { d.push_back(d.root(), source.root()["key"]); }) == bad_value);
+    CHECK(exception_of_call([&] { d.set(d.root()[0], source.root()["list"][0]); }) == bad_value);
+    CHECK(exception_of_call([&] { d.set(d.root(), 0, source.root()["list"]); }) == bad_value);
+    CHECK(exception_of_call([&] { d.set(d.root(), 0, source.root()); }) == bad_value);
+
+    text[text.find("key") + 1] = '\xC3'; // a key is checked as well
+    const std::string bad_key = message_of(std::string("k\xC3" "y"));
+    CHECK(exception_of_call([&] { d.set(d.root(), 0, source.root()); }) == bad_key);
+    CHECK(exception_of_call([&] { d.push_back(d.root(), source.root()); }) == bad_key);
+
+    // nothing of the failed edits is visible
+    CHECK(d.root().dump() == R"([1,{"key":"abc","list":["abc"]}])");
+}
+#endif

tests/src/unit-merge_patch.cpp

@@ -46,7 +46,7 @@ void reference_merge_patch(json& target, const json& patch)
 
 // objects nested `depth` levels deep under the key "a", with members that
 // differ by `variant` on the way down
-std::string nested_objects(const std::size_t depth, const int variant)
+std::string nested_objects_merge_patch(const std::size_t depth, const int variant)
 {
     std::string text;
     for (std::size_t i = 0; i < depth; ++i)
@@ -309,9 +309,9 @@ TEST_CASE("JSON Merge Patch on deeply nested values")
             for (int variant = 0; variant < 3; ++variant)
             {
                 CAPTURE(variant)
-                const json patch = json::parse(nested_objects(depth, variant));
+                const json patch = json::parse(nested_objects_merge_patch(depth, variant));
 
-                json result = json::parse(nested_objects(depth, (variant + 1) % 3));
+                json result = json::parse(nested_objects_merge_patch(depth, (variant + 1) % 3));
                 json expected = result;
                 result.merge_patch(patch);
                 reference_merge_patch(expected, patch);
@@ -332,8 +332,8 @@ TEST_CASE("JSON Merge Patch on deeply nested values")
         // applying a patch used to recurse once per nesting level. The result
         // is only walked, never copied or compared, since those recurse too.
         const std::size_t depth = 100000;
-        json target = json::parse(nested_objects(depth, 0));
-        target.merge_patch(json::parse(nested_objects(depth, 1)));
+        json target = json::parse(nested_objects_merge_patch(depth, 0));
+        target.merge_patch(json::parse(nested_objects_merge_patch(depth, 1)));
 
         const json* p = &target;
         for (std::size_t i = 0; i < depth; ++i)
@@ -404,7 +404,7 @@ TEST_CASE("merge_patch() with an argument that aliases *this (#5641)")
                 })
         {
             CAPTURE(depth)
-            json j = json::parse(nested_objects(depth, 0));
+            json j = json::parse(nested_objects_merge_patch(depth, 0));
             const json expected = j;
             j.merge_patch(j);
             CHECK(j == expected);

tests/src/unit-modifiers.cpp

@@ -35,7 +35,7 @@ void reference_update(json& target, const json& source)
 
 // objects nested `depth` levels deep under the key "a", with members that
 // differ by `variant` on the way down
-std::string nested_objects(const std::size_t depth, const int variant)
+std::string nested_objects_update(const std::size_t depth, const int variant)
 {
     std::string text;
     for (std::size_t i = 0; i < depth; ++i)
@@ -1131,8 +1131,8 @@ TEST_CASE("update() on deeply nested values")
             for (int variant = 0; variant < 3; ++variant)
             {
                 CAPTURE(variant)
-                const json source = json::parse(nested_objects(depth, variant));
-                json result = json::parse(nested_objects(depth, (variant + 1) % 3));
+                const json source = json::parse(nested_objects_update(depth, variant));
+                json result = json::parse(nested_objects_update(depth, (variant + 1) % 3));
                 json expected = result;
                 result.update(source, true);
                 reference_update(expected, source);
@@ -1146,8 +1146,8 @@ TEST_CASE("update() on deeply nested values")
         // merging used to recurse once per nesting level. The result is only
         // walked, never copied or compared, since those recurse too.
         const std::size_t depth = 100000;
-        json target = json::parse(nested_objects(depth, 0));
-        target.update(json::parse(nested_objects(depth, 1)), true);
+        json target = json::parse(nested_objects_update(depth, 0));
+        target.update(json::parse(nested_objects_update(depth, 1)), true);
 
         const json* p = &target;
         for (std::size_t i = 0; i < depth; ++i)
@@ -1223,7 +1223,7 @@ TEST_CASE("update() with an argument that aliases *this (#5641)")
                 })
         {
             CAPTURE(depth)
-            json j = json::parse(nested_objects(depth, 0));
+            json j = json::parse(nested_objects_update(depth, 0));
             const json expected = j;
             j.update(j, true);
             CHECK(j == expected);

tests/src/unit-msgpack-custom-types.cpp

@@ -0,0 +1,296 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// MessagePack tests with basic_json specializations other than nlohmann::json;
+// they are kept apart from unit-msgpack.cpp because every specialization adds
+// many sections to the object file, and MinGW cannot link objects with more
+// than 65535 sections (see the clang job in .github/workflows/windows.yml)
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#include <cstdint> // SIZE_MAX, UINT32_MAX
+#include <functional>
+#include <limits>
+#include <map>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+// the fake sizes below do not fit into a 32-bit std::size_t
+// with clang and libstdc++ 10, the std::filesystem::path conversion that
+// C++17 builds consider for every string type is ambiguous for a class
+// derived from std::string, so the string case is not tested there
+#if !(defined(__clang__) && defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 11)
+    #define JSON_TEST_BEYOND_UINT32_STRING 1
+#endif
+
+#if SIZE_MAX > UINT32_MAX
+template<typename T, typename A = std::allocator<T>>
+struct msgpack_huge_array : std::vector<T, A>
+{
+    using base = std::vector<T, A>;
+    using base::base;
+
+    bool fake_size = false;
+
+    std::size_t size() const noexcept
+    {
+        if (fake_size)
+        {
+            return (std::numeric_limits<std::uint32_t>::max)() + 1ULL;
+        }
+
+        return base::size();
+    }
+};
+
+using msgpack_huge_array_json = nlohmann::json::with_array_t<msgpack_huge_array>;
+
+TEST_CASE("MessagePack Size above uint32 for array")
+{
+    msgpack_huge_array_json j = msgpack_huge_array_json::array();
+
+    j.push_back(1);
+    j.push_back(2);
+    j.push_back(3);
+
+    auto& array = j.get_ref<msgpack_huge_array_json::array_t&>();
+    array.fake_size = true;
+
+    // write into a caller-owned vector: to_msgpack(j) reserves space based on
+    // the (faked) element count, which fails with bad_alloc on Windows
+    std::vector<std::uint8_t> result;
+    CHECK_THROWS_WITH_AS(
+        msgpack_huge_array_json::to_msgpack(j, result),
+        "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
+        json::out_of_range&);
+
+    array.fake_size = false;
+}
+
+template<typename K, typename V,
+         typename C = std::less<K>,
+         typename A = std::allocator<std::pair<const K, V>>>
+                                     struct msgpack_huge_map : std::map<K, V, C, A>
+{
+    using base = std::map<K, V, C, A>;
+    using base::base;
+
+    bool fake_size = false;
+
+    std::size_t size() const noexcept
+    {
+        if (fake_size)
+        {
+            return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
+        }
+
+        return base::size();
+    }
+};
+
+using msgpack_huge_object_json = nlohmann::json::with_object_t<msgpack_huge_map>;
+
+TEST_CASE("MessagePack Size above uint32 for object")
+{
+
+    msgpack_huge_object_json j = msgpack_huge_object_json::object();
+
+    j["one"] = 1;
+    j["two"] = 2;
+
+    auto& object = j.get_ref<msgpack_huge_object_json::object_t&>();
+    object.fake_size = true;
+
+    // write into a caller-owned vector: to_msgpack(j) reserves space based on
+    // the (faked) element count, which fails with bad_alloc on Windows
+    std::vector<std::uint8_t> result;
+    CHECK_THROWS_WITH_AS(
+        msgpack_huge_object_json::to_msgpack(j, result),
+        "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
+        json::out_of_range&);
+
+    object.fake_size = false;
+}
+
+#ifdef JSON_TEST_BEYOND_UINT32_STRING
+struct msgpack_huge_string : std::string
+{
+    using std::string::string;
+
+    std::size_t size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
+    {
+        return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
+    }
+};
+
+using msgpack_huge_string_json = nlohmann::json::with_string_t<msgpack_huge_string>;
+
+TEST_CASE("MessagePack Size above uint32 for string")
+{
+    const msgpack_huge_string_json j = "hello";
+
+    CHECK_THROWS_WITH_AS(
+        msgpack_huge_string_json::to_msgpack(j),
+        "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
+        json::out_of_range&);
+}
+#endif
+
+struct msgpack_huge_binary : std::vector<std::uint8_t>
+{
+    using std::vector<std::uint8_t>::vector;
+
+    std::size_t size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
+    {
+        return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
+    }
+};
+
+using msgpack_huge_binary_json = nlohmann::json::with_binary_t<msgpack_huge_binary>;
+
+TEST_CASE("MessagePack Size above uint32 for binary")
+{
+
+    msgpack_huge_binary_json j = msgpack_huge_binary_json::binary(msgpack_huge_binary{});
+
+    j.get_binary().push_back(0x01);
+    j.get_binary().push_back(0x02);
+
+    CHECK_THROWS_WITH_AS(
+        msgpack_huge_binary_json::to_msgpack(j),
+        "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
+        json::out_of_range&);
+}
+#endif
+
+namespace
+{
+// types that report a size beyond UINT32_MAX without allocating that much
+// memory, so the MessagePack length limit can be tested cheaply; see the
+// similar types in unit-bson.cpp
+std::size_t beyond_uint32_size()
+{
+    return static_cast<std::size_t>((std::numeric_limits<std::uint32_t>::max)()) + 1;
+}
+
+class beyond_uint32_binary_t : public std::vector<std::uint8_t>
+{
+  public:
+    using std::vector<std::uint8_t>::vector;
+
+    size_type size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
+    {
+        return beyond_uint32_size();
+    }
+};
+
+#ifdef JSON_TEST_BEYOND_UINT32_STRING
+class beyond_uint32_string_t : public std::string
+{
+  public:
+    using std::string::string;
+
+    size_type size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
+    {
+        return beyond_uint32_size();
+    }
+};
+
+using beyond_uint32_string_json = nlohmann::json::with_string_t<beyond_uint32_string_t>;
+#endif
+
+using beyond_uint32_binary_json = nlohmann::json::with_binary_t<beyond_uint32_binary_t>;
+} // namespace
+
+TEST_CASE("MessagePack lengths beyond UINT32_MAX cannot be serialized")
+{
+    // MessagePack stores the length of a string, binary value, array, or
+    // object in at most 32 bits; a larger one used to be written without any
+    // length at all
+#if SIZE_MAX > UINT32_MAX
+    {
+        const char* const expected = "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295";
+
+        const beyond_uint32_binary_json binary = beyond_uint32_binary_json::binary(beyond_uint32_binary_t{});
+        CHECK_THROWS_WITH_AS(beyond_uint32_binary_json::to_msgpack(binary), expected, beyond_uint32_binary_json::out_of_range&);
+
+        const beyond_uint32_binary_json ext = beyond_uint32_binary_json::binary(beyond_uint32_binary_t{}, 42);
+        CHECK_THROWS_WITH_AS(beyond_uint32_binary_json::to_msgpack(ext), expected, beyond_uint32_binary_json::out_of_range&);
+
+#ifdef JSON_TEST_BEYOND_UINT32_STRING
+        // created from its type rather than from a beyond_uint32_string_t:
+        // that would consider the std::filesystem::path conversion, which
+        // libstdc++ 10 cannot decide for a class derived from std::string
+        const beyond_uint32_string_json string(beyond_uint32_string_json::value_t::string);
+        CHECK_THROWS_WITH_AS(beyond_uint32_string_json::to_msgpack(string), expected, beyond_uint32_string_json::out_of_range&);
+#endif
+    }
+#endif
+}
+
+TEST_CASE("MessagePack numbers use the active union member (see #5644)")
+{
+    // when number_integer_t is narrower than number_unsigned_t, to_msgpack()
+    // used to read the union member that was not the active one, writing
+    // wrong bytes for some values; std::int64_t/std::uint64_t (the default
+    // types, where both members have the same width) were not affected
+    using int32_json = nlohmann::json::with_integers_t<std::int32_t, std::uint64_t>;
+    using int16_json = nlohmann::json::with_integers_t<std::int16_t, std::uint64_t>;
+
+    SECTION("number_integer_t = std::int32_t")
+    {
+        SECTION("6442450944 (uint 64; the low 32 bits used to be sign-extended)")
+        {
+            const int32_json j = 6442450944ULL;
+            CHECK(j.is_number_unsigned());
+
+            std::vector<uint8_t> const expected{0xcf, 0x00, 0x00, 0x00, 0x01, 0x80, 0x00, 0x00, 0x00};
+            const auto result = int32_json::to_msgpack(j);
+            CHECK(result == expected);
+            CHECK(int32_json::from_msgpack(result) == j);
+        }
+
+        SECTION("4294967496 (uint 64; the low 32 bits used to be the whole value)")
+        {
+            const int32_json j = 4294967496ULL;
+            CHECK(j.is_number_unsigned());
+
+            std::vector<uint8_t> const expected{0xcf, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0xc8};
+            const auto result = int32_json::to_msgpack(j);
+            CHECK(result == expected);
+            CHECK(int32_json::from_msgpack(result) == j);
+        }
+    }
+
+    SECTION("number_integer_t = std::int16_t, 98304 (uint 32)")
+    {
+        const int16_json j = 98304ULL;
+        CHECK(j.is_number_unsigned());
+
+        std::vector<uint8_t> const expected{0xce, 0x00, 0x01, 0x80, 0x00};
+        const auto result = int16_json::to_msgpack(j);
+        CHECK(result == expected);
+        CHECK(int16_json::from_msgpack(result) == j);
+    }
+
+    SECTION("default types (std::int64_t/std::uint64_t) are unaffected")
+    {
+        const json j = 4294967496ULL;
+        CHECK(j.is_number_unsigned());
+
+        std::vector<uint8_t> const expected{0xcf, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0xc8};
+        const auto result = json::to_msgpack(j);
+        CHECK(result == expected);
+        CHECK(json::from_msgpack(result) == j);
+    }
+}

tests/src/unit-msgpack.cpp

@@ -2173,276 +2173,6 @@ TEST_CASE("MessagePack roundtrips" * doctest::skip())
     }
 }
 
-// the fake sizes below do not fit into a 32-bit std::size_t
-// with clang and libstdc++ 10, the std::filesystem::path conversion that
-// C++17 builds consider for every string type is ambiguous for a class
-// derived from std::string, so the string case is not tested there
-#if !(defined(__clang__) && defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 11)
-    #define JSON_TEST_BEYOND_UINT32_STRING 1
-#endif
-
-#if SIZE_MAX > UINT32_MAX
-template<typename T, typename A = std::allocator<T>>
-struct huge_array : std::vector<T, A>
-{
-    using base = std::vector<T, A>;
-    using base::base;
-
-    bool fake_size = false;
-
-    std::size_t size() const noexcept
-    {
-        if (fake_size)
-        {
-            return (std::numeric_limits<std::uint32_t>::max)() + 1ULL;
-        }
-
-        return base::size();
-    }
-};
-
-using huge_array_json = nlohmann::json::with_array_t<huge_array>;
-
-TEST_CASE("MessagePack Size above uint32 for array")
-{
-    huge_array_json j = huge_array_json::array();
-
-    j.push_back(1);
-    j.push_back(2);
-    j.push_back(3);
-
-    auto& array = j.get_ref<huge_array_json::array_t&>();
-    array.fake_size = true;
-
-    // write into a caller-owned vector: to_msgpack(j) reserves space based on
-    // the (faked) element count, which fails with bad_alloc on Windows
-    std::vector<std::uint8_t> result;
-    CHECK_THROWS_WITH_AS(
-        huge_array_json::to_msgpack(j, result),
-        "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
-        json::out_of_range&);
-
-    array.fake_size = false;
-}
-
-template<typename K, typename V,
-         typename C = std::less<K>,
-         typename A = std::allocator<std::pair<const K, V>>>
-                                     struct huge_map : std::map<K, V, C, A>
-{
-    using base = std::map<K, V, C, A>;
-    using base::base;
-
-    bool fake_size = false;
-
-    std::size_t size() const noexcept
-    {
-        if (fake_size)
-        {
-            return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
-        }
-
-        return base::size();
-    }
-};
-
-using huge_object_json = nlohmann::json::with_object_t<huge_map>;
-
-TEST_CASE("MessagePack Size above uint32 for object")
-{
-
-    huge_object_json j = huge_object_json::object();
-
-    j["one"] = 1;
-    j["two"] = 2;
-
-    auto& object = j.get_ref<huge_object_json::object_t&>();
-    object.fake_size = true;
-
-    // write into a caller-owned vector: to_msgpack(j) reserves space based on
-    // the (faked) element count, which fails with bad_alloc on Windows
-    std::vector<std::uint8_t> result;
-    CHECK_THROWS_WITH_AS(
-        huge_object_json::to_msgpack(j, result),
-        "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
-        json::out_of_range&);
-
-    object.fake_size = false;
-}
-
-#ifdef JSON_TEST_BEYOND_UINT32_STRING
-struct huge_string : std::string
-{
-    using std::string::string;
-
-    std::size_t size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
-    {
-        return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
-    }
-};
-
-using huge_string_json = nlohmann::json::with_string_t<huge_string>;
-
-TEST_CASE("MessagePack Size above uint32 for string")
-{
-    const huge_string_json j = "hello";
-
-    CHECK_THROWS_WITH_AS(
-        huge_string_json::to_msgpack(j),
-        "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
-        json::out_of_range&);
-}
-#endif
-
-struct huge_binary : std::vector<std::uint8_t>
-{
-    using std::vector<std::uint8_t>::vector;
-
-    std::size_t size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
-    {
-        return static_cast<std::size_t>(UINT32_MAX) + 1ULL;
-    }
-};
-
-using huge_binary_json = nlohmann::json::with_binary_t<huge_binary>;
-
-TEST_CASE("MessagePack Size above uint32 for binary")
-{
-
-    huge_binary_json j = huge_binary_json::binary(huge_binary{});
-
-    j.get_binary().push_back(0x01);
-    j.get_binary().push_back(0x02);
-
-    CHECK_THROWS_WITH_AS(
-        huge_binary_json::to_msgpack(j),
-        "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295",
-        json::out_of_range&);
-}
-#endif
-
-namespace
-{
-// types that report a size beyond UINT32_MAX without allocating that much
-// memory, so the MessagePack length limit can be tested cheaply; see the
-// similar types in unit-bson.cpp
-std::size_t beyond_uint32_size()
-{
-    return static_cast<std::size_t>((std::numeric_limits<std::uint32_t>::max)()) + 1;
-}
-
-class beyond_uint32_binary_t : public std::vector<std::uint8_t>
-{
-  public:
-    using std::vector<std::uint8_t>::vector;
-
-    size_type size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
-    {
-        return beyond_uint32_size();
-    }
-};
-
-#ifdef JSON_TEST_BEYOND_UINT32_STRING
-class beyond_uint32_string_t : public std::string
-{
-  public:
-    using std::string::string;
-
-    size_type size() const noexcept // NOLINT(readability-convert-member-functions-to-static)
-    {
-        return beyond_uint32_size();
-    }
-};
-
-using beyond_uint32_string_json = nlohmann::json::with_string_t<beyond_uint32_string_t>;
-#endif
-
-using beyond_uint32_binary_json = nlohmann::json::with_binary_t<beyond_uint32_binary_t>;
-} // namespace
-
-TEST_CASE("MessagePack lengths beyond UINT32_MAX cannot be serialized")
-{
-    // MessagePack stores the length of a string, binary value, array, or
-    // object in at most 32 bits; a larger one used to be written without any
-    // length at all
-#if SIZE_MAX > UINT32_MAX
-    {
-        const char* const expected = "[json.exception.out_of_range.412] MessagePack length 4294967296 exceeds maximum of 4294967295";
-
-        const beyond_uint32_binary_json binary = beyond_uint32_binary_json::binary(beyond_uint32_binary_t{});
-        CHECK_THROWS_WITH_AS(beyond_uint32_binary_json::to_msgpack(binary), expected, beyond_uint32_binary_json::out_of_range&);
-
-        const beyond_uint32_binary_json ext = beyond_uint32_binary_json::binary(beyond_uint32_binary_t{}, 42);
-        CHECK_THROWS_WITH_AS(beyond_uint32_binary_json::to_msgpack(ext), expected, beyond_uint32_binary_json::out_of_range&);
-
-#ifdef JSON_TEST_BEYOND_UINT32_STRING
-        // created from its type rather than from a beyond_uint32_string_t:
-        // that would consider the std::filesystem::path conversion, which
-        // libstdc++ 10 cannot decide for a class derived from std::string
-        const beyond_uint32_string_json string(beyond_uint32_string_json::value_t::string);
-        CHECK_THROWS_WITH_AS(beyond_uint32_string_json::to_msgpack(string), expected, beyond_uint32_string_json::out_of_range&);
-#endif
-    }
-#endif
-}
-
-TEST_CASE("MessagePack numbers use the active union member (see #5644)")
-{
-    // when number_integer_t is narrower than number_unsigned_t, to_msgpack()
-    // used to read the union member that was not the active one, writing
-    // wrong bytes for some values; std::int64_t/std::uint64_t (the default
-    // types, where both members have the same width) were not affected
-    using int32_json = nlohmann::json::with_integers_t<std::int32_t, std::uint64_t>;
-    using int16_json = nlohmann::json::with_integers_t<std::int16_t, std::uint64_t>;
-
-    SECTION("number_integer_t = std::int32_t")
-    {
-        SECTION("6442450944 (uint 64; the low 32 bits used to be sign-extended)")
-        {
-            const int32_json j = 6442450944ULL;
-            CHECK(j.is_number_unsigned());
-
-            std::vector<uint8_t> const expected{0xcf, 0x00, 0x00, 0x00, 0x01, 0x80, 0x00, 0x00, 0x00};
-            const auto result = int32_json::to_msgpack(j);
-            CHECK(result == expected);
-            CHECK(int32_json::from_msgpack(result) == j);
-        }
-
-        SECTION("4294967496 (uint 64; the low 32 bits used to be the whole value)")
-        {
-            const int32_json j = 4294967496ULL;
-            CHECK(j.is_number_unsigned());
-
-            std::vector<uint8_t> const expected{0xcf, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0xc8};
-            const auto result = int32_json::to_msgpack(j);
-            CHECK(result == expected);
-            CHECK(int32_json::from_msgpack(result) == j);
-        }
-    }
-
-    SECTION("number_integer_t = std::int16_t, 98304 (uint 32)")
-    {
-        const int16_json j = 98304ULL;
-        CHECK(j.is_number_unsigned());
-
-        std::vector<uint8_t> const expected{0xce, 0x00, 0x01, 0x80, 0x00};
-        const auto result = int16_json::to_msgpack(j);
-        CHECK(result == expected);
-        CHECK(int16_json::from_msgpack(result) == j);
-    }
-
-    SECTION("default types (std::int64_t/std::uint64_t) are unaffected")
-    {
-        const json j = 4294967496ULL;
-        CHECK(j.is_number_unsigned());
-
-        std::vector<uint8_t> const expected{0xcf, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0xc8};
-        const auto result = json::to_msgpack(j);
-        CHECK(result == expected);
-        CHECK(json::from_msgpack(result) == j);
-    }
-}
-
 TEST_CASE("MessagePack large strings and binaries (chunked reader)")
 {
     // get_msgpack_string()/get_msgpack_binary() both read through get_binary(),

tests/src/unit-ordered_json2.cpp

@@ -44,6 +44,8 @@ using nlohmann::ordered_json;
 // this type, which is presumably why the gap was never noticed.
 /////////////////////////////////////////////////////////////////////////////
 
+namespace unit_ordered_json2_detail
+{
 class alt_string;
 bool operator<(const char* op1, const alt_string& op2) noexcept; // NOLINT(misc-use-internal-linkage)
 void int_to_string(alt_string& target, std::size_t value); // NOLINT(misc-use-internal-linkage)
@@ -217,12 +219,15 @@ void int_to_string(alt_string& target, std::size_t value)
     target = std::to_string(value).c_str();
 }
 
-using alt_json = nlohmann::json::with_string_t<alt_string>;
+using ordered_json2_alt_json = nlohmann::json::with_string_t<alt_string>;
 
 bool operator<(const char* op1, const alt_string& op2) noexcept
 {
     return op1 < op2.str_impl;
 }
+} // namespace unit_ordered_json2_detail
+
+using unit_ordered_json2_detail::ordered_json2_alt_json;
 
 namespace
 {
@@ -253,11 +258,11 @@ ordered_json make_rich_ordered_json()
     return j;
 }
 
-alt_json make_rich_alt_json()
+ordered_json2_alt_json make_rich_alt_json()
 {
-    alt_json j;
+    ordered_json2_alt_json j;
     j["zebra"] = 1;
-    j["apple"] = alt_json::array({1, 2, 3});
+    j["apple"] = ordered_json2_alt_json::array({1, 2, 3});
     j["mango"]["z_nested"] = true;
     j["mango"]["a_nested"] = nullptr;
     j["banana"] = "some text";
@@ -330,47 +335,47 @@ TEST_CASE("ordered_json across binary formats")
 
 TEST_CASE("alt_json (custom string_t) across binary formats")
 {
-    const alt_json original = make_rich_alt_json();
+    const ordered_json2_alt_json original = make_rich_alt_json();
 
     SECTION("CBOR")
     {
-        const auto bytes = alt_json::to_cbor(original);
-        const auto restored = alt_json::from_cbor(bytes);
+        const auto bytes = ordered_json2_alt_json::to_cbor(original);
+        const auto restored = ordered_json2_alt_json::from_cbor(bytes);
         CHECK(restored == original);
     }
 
     SECTION("MessagePack")
     {
-        const auto bytes = alt_json::to_msgpack(original);
-        const auto restored = alt_json::from_msgpack(bytes);
+        const auto bytes = ordered_json2_alt_json::to_msgpack(original);
+        const auto restored = ordered_json2_alt_json::from_msgpack(bytes);
         CHECK(restored == original);
     }
 
     SECTION("UBJSON")
     {
-        const auto bytes = alt_json::to_ubjson(original);
-        const auto restored = alt_json::from_ubjson(bytes);
+        const auto bytes = ordered_json2_alt_json::to_ubjson(original);
+        const auto restored = ordered_json2_alt_json::from_ubjson(bytes);
         CHECK(restored == original);
     }
 
     SECTION("BON8")
     {
-        const auto bytes = alt_json::to_bon8(original);
-        const auto restored = alt_json::from_bon8(bytes);
+        const auto bytes = ordered_json2_alt_json::to_bon8(original);
+        const auto restored = ordered_json2_alt_json::from_bon8(bytes);
         CHECK(restored == original);
     }
 
     SECTION("BSON")
     {
-        const auto bytes = alt_json::to_bson(original);
-        const auto restored = alt_json::from_bson(bytes);
+        const auto bytes = ordered_json2_alt_json::to_bson(original);
+        const auto restored = ordered_json2_alt_json::from_bson(bytes);
         CHECK(restored == original);
     }
 
     SECTION("BJData")
     {
-        const auto bytes = alt_json::to_bjdata(original);
-        const auto restored = alt_json::from_bjdata(bytes);
+        const auto bytes = ordered_json2_alt_json::to_bjdata(original);
+        const auto restored = ordered_json2_alt_json::from_bjdata(bytes);
         CHECK(restored == original);
     }
 }

tests/src/unit-ordered_map-cpp17.cpp

@@ -0,0 +1,40 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-ordered_map.cpp (ordered_map::find with
+// std::string_view keys). It is kept in a separate translation unit so the (much larger)
+// unit-ordered_map.cpp is built for C++11 only and not rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::ordered_map;
+
+#ifdef JSON_HAS_CPP_17
+#include <string>
+#include <string_view>
+
+TEST_CASE("ordered_map (C++17)")
+{
+    SECTION("find")
+    {
+        ordered_map<std::string, std::string> om;
+        om["eins"] = "one";
+        om["zwei"] = "two";
+        om["drei"] = "three";
+        const auto com = om;
+
+        const std::string eins("eins");
+        const std::string vier("vier");
+
+        CHECK(om.find(std::string_view("eins")) == om.begin());
+        CHECK(com.find(std::string_view("eins")) == com.begin());
+    }
+}
+
+#endif

tests/src/unit-ordered_map.cpp

@@ -360,11 +360,6 @@ TEST_CASE("ordered_map")
         CHECK(com.find("vier") == com.end());
         CHECK(com.find(std::string("vier")) == com.end());
         CHECK(com.find(vier) == com.end());
-
-#ifdef JSON_HAS_CPP_17
-        CHECK(om.find(std::string_view("eins")) == om.begin());
-        CHECK(com.find(std::string_view("eins")) == com.begin());
-#endif
     }
 
     SECTION("insert")

tests/src/unit-regression2-cpp17.cpp

@@ -0,0 +1,67 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-regression2.cpp (std::variant, std::any
+// and std::optional regression tests). It is kept in a separate translation unit so the
+// (much larger) unit-regression2.cpp is built for C++11 only and not rebuilt for every C++
+// standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using json = nlohmann::json;
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
+
+#ifdef JSON_HAS_CPP_17
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#include <any>
+#include <variant>
+
+#if __has_include(<optional>)
+    #include <optional>
+#elif __has_include(<experimental/optional>)
+    #include <experimental/optional>
+#endif
+
+/////////////////////////////////////////////////////////////////////
+// for #4804
+/////////////////////////////////////////////////////////////////////
+using json_4804 = nlohmann::json::with_binary_t<std::vector<std::byte>>;
+
+TEST_CASE("regression tests 2 (C++17)")
+{
+    SECTION("issue #1292 - Serializing std::variant causes stack overflow")
+    {
+        static_assert(!std::is_constructible<json, std::variant<int, float>>::value, "unexpected value");
+    }
+
+    SECTION("issue #5066 - MSVC converts json to std::variant<json> via the conversion operator")
+    {
+        // std::variant<json> must not be retrievable via get<>(), because otherwise the
+        // implicit conversion operator becomes a candidate that MSVC picks over the variant's
+        // converting constructor, routing a number through the string from_json overload
+        static_assert(!nlohmann::detail::is_detected<nlohmann::detail::get_template_function, const json&, std::variant<json>>::value,
+                      "std::variant<json> must not be retrievable via get<>()");
+
+        // clang before 7 cannot instantiate libstdc++'s std::variant<json>
+#if !(defined(__clang__) && __clang_major__ < 7)
+        // push_back, not emplace_back: #5066 needs the implicit conversion
+        // from json to the vector's value type
+        std::vector<std::variant<json>> v;
+        v.push_back(json(1)); // NOLINT(hicpp-use-emplace,modernize-use-emplace)
+        CHECK(std::get<0>(v[0]) == 1);
+#endif
+    }
+}
+
+#endif

tests/src/unit-regression2-cpp20.cpp

@@ -0,0 +1,57 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++20-only part of unit-regression2.cpp (std::span and ranges
+// regression tests). It is kept in a separate translation unit so the (much larger) unit-
+// regression2.cpp is built for C++11 only and not rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using json = nlohmann::json;
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
+
+#ifdef JSON_HAS_CPP_20
+#include <string>
+
+#if __has_include(<span>)
+    #include <span>
+#endif
+
+/////////////////////////////////////////////////////////////////////
+// for #4440
+/////////////////////////////////////////////////////////////////////
+#if JSON_HAS_RANGES == 1
+    #include <ranges>
+#endif
+
+TEST_CASE("regression tests 2 (C++20)")
+{
+#ifndef _LIBCPP_VERSION // see https://github.com/nlohmann/json/issues/4490
+    // classic Intel ICC reports <span> as includable but cannot actually compile
+    // std::span/std::as_bytes usage below
+#if __has_include(<span>) && !defined(__ICC) && !defined(__INTEL_COMPILER)
+    SECTION("issue #2546 - parsing containers of std::byte")
+    {
+        const char DATA[] = R"("Hello, world!")"; // NOLINT(misc-const-correctness,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
+        // exclude the trailing '\0' that string-literal initialization adds to
+        // DATA: std::span(DATA) would span the full array extent (including
+        // that NUL), which is only silently accepted as end-of-input by default
+        // and would fail under JSON_STRICT_NUL_HANDLING
+        const auto s = std::as_bytes(std::span(DATA, sizeof(DATA) - 1));
+        const json j = json::parse(s);
+        CHECK(j.dump() == "\"Hello, world!\"");
+    }
+#endif
+#endif
+
+}
+
+#endif

tests/src/unit-regression2.cpp

@@ -6,12 +6,11 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// When using macros that are only defined for particular versions of the standard
-// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
-// version macro in a comment close by, like this:
-// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. To avoid rebuilding this
+// large file for every standard, tests that depend on the standard version (e.g.,
+// those using JSON_HAS_FILESYSTEM, JSON_HAS_RANGES, or JSON_HAS_THREE_WAY_COMPARISON)
+// go into a separate file unit-regression2-cpp<NN>.cpp. This file stays C++11-only.
 
 #include "doctest_compatibility.h"
 
@@ -49,30 +48,6 @@ using ordered_json = nlohmann::ordered_json;
 
 #include "test_utils.hpp"
 
-#ifdef JSON_HAS_CPP_17
-    #include <any>
-    #include <variant>
-#endif
-
-#ifdef JSON_HAS_CPP_17
-    #if __has_include(<optional>)
-        #include <optional>
-    #elif __has_include(<experimental/optional>)
-        #include <experimental/optional>
-    #endif
-
-    /////////////////////////////////////////////////////////////////////
-    // for #4804
-    /////////////////////////////////////////////////////////////////////
-    using json_4804 = nlohmann::json::with_binary_t<std::vector<std::byte>>;
-#endif
-
-#ifdef JSON_HAS_CPP_20
-    #if __has_include(<span>)
-        #include <span>
-    #endif
-#endif
-
 /////////////////////////////////////////////////////////////////////
 // for #4825 - explicitly instantiating basic_json must compile; this
 // forces instantiation of binary_writer::write_bjdata_ndarray, whose
@@ -81,13 +56,6 @@ using ordered_json = nlohmann::ordered_json;
 /////////////////////////////////////////////////////////////////////
 template class nlohmann::basic_json<>;
 
-/////////////////////////////////////////////////////////////////////
-// for #4440
-/////////////////////////////////////////////////////////////////////
-#if JSON_HAS_RANGES == 1
-    #include <ranges>
-#endif
-
 // NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
 DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
@@ -285,18 +253,18 @@ struct adl_serializer<NonDefaultConstructible>
 /////////////////////////////////////////////////////////////////////
 
 template<class T>
-class my_allocator : public std::allocator<T>
+class my_allocator_2982 : public std::allocator<T>
 {
   public:
     using std::allocator<T>::allocator;
 
-    my_allocator() = default;
-    template<class U> my_allocator(const my_allocator<U>& /*unused*/) { }
+    my_allocator_2982() = default;
+    template<class U> my_allocator_2982(const my_allocator_2982<U>& /*unused*/) { }
 
     template <class U>
     struct rebind
     {
-        using other = my_allocator<U>;
+        using other = my_allocator_2982<U>;
     };
 };
 
@@ -468,13 +436,6 @@ TEST_CASE("regression tests 2")
         CHECK(diffs.size() == 1);  // Note the change here, was 2
     }
 
-#ifdef JSON_HAS_CPP_17
-    SECTION("issue #1292 - Serializing std::variant causes stack overflow")
-    {
-        static_assert(!std::is_constructible<json, std::variant<int, float>>::value, "unexpected value");
-    }
-#endif
-
     SECTION("issue #1299 - compile error in from_json converting to container "
             "with std::pair")
     {
@@ -718,26 +679,6 @@ TEST_CASE("regression tests 2")
         CHECK(j.dump() == "{}");
     }
 
-#ifdef JSON_HAS_CPP_20
-#ifndef _LIBCPP_VERSION // see https://github.com/nlohmann/json/issues/4490
-    // classic Intel ICC reports <span> as includable but cannot actually compile
-    // std::span/std::as_bytes usage below
-#if __has_include(<span>) && !defined(__ICC) && !defined(__INTEL_COMPILER)
-    SECTION("issue #2546 - parsing containers of std::byte")
-    {
-        const char DATA[] = R"("Hello, world!")"; // NOLINT(misc-const-correctness,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
-        // exclude the trailing '\0' that string-literal initialization adds to
-        // DATA: std::span(DATA) would span the full array extent (including
-        // that NUL), which is only silently accepted as end-of-input by default
-        // and would fail under JSON_STRICT_NUL_HANDLING
-        const auto s = std::as_bytes(std::span(DATA, sizeof(DATA) - 1));
-        const json j = json::parse(s);
-        CHECK(j.dump() == "\"Hello, world!\"");
-    }
-#endif
-#endif
-#endif
-
     SECTION("issue #2574 - Deserialization to std::array, std::pair, and std::tuple with non-default constructable types fails")
     {
         SECTION("std::array")
@@ -868,7 +809,7 @@ TEST_CASE("regression tests 2")
 
     SECTION("issue #2982 - to_{binary format} does not provide a mechanism for specifying a custom allocator for the returned type")
     {
-        std::vector<std::uint8_t, my_allocator<std::uint8_t>> my_vector;
+        std::vector<std::uint8_t, my_allocator_2982<std::uint8_t>> my_vector;
         const json j = {1, 2, 3, 4};
         json::to_cbor(j, my_vector);
         json k = json::from_cbor(my_vector);
@@ -884,26 +825,6 @@ TEST_CASE("regression tests 2")
         CHECK(node.dump(-1, ' ', true, json::error_handler_t::keep) == "{\"test\":\"test\334\\u0005\"}");
     }
 
-#ifdef JSON_HAS_CPP_17
-    SECTION("issue #5066 - MSVC converts json to std::variant<json> via the conversion operator")
-    {
-        // std::variant<json> must not be retrievable via get<>(), because otherwise the
-        // implicit conversion operator becomes a candidate that MSVC picks over the variant's
-        // converting constructor, routing a number through the string from_json overload
-        static_assert(!nlohmann::detail::is_detected<nlohmann::detail::get_template_function, const json&, std::variant<json>>::value,
-                      "std::variant<json> must not be retrievable via get<>()");
-
-        // clang before 7 cannot instantiate libstdc++'s std::variant<json>
-#if !(defined(__clang__) && __clang_major__ < 7)
-        // push_back, not emplace_back: #5066 needs the implicit conversion
-        // from json to the vector's value type
-        std::vector<std::variant<json>> v;
-        v.push_back(json(1)); // NOLINT(hicpp-use-emplace,modernize-use-emplace)
-        CHECK(std::get<0>(v[0]) == 1);
-#endif
-    }
-#endif
-
     SECTION("issue #3669 - invalid use of incomplete type with optional member and to_json")
     {
         const Issue3669Holder h{};

tests/src/unit-regression3-cpp17.cpp

@@ -0,0 +1,115 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++17-only part of unit-regression3.cpp (std::any, std::optional,
+// std::variant and std::filesystem regression tests). It is kept in a separate translation
+// unit so the (much larger) unit-regression3.cpp is built for C++11 only and not rebuilt
+// for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using json = nlohmann::json;
+using ordered_json = nlohmann::ordered_json;
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
+
+#ifdef JSON_HAS_CPP_17
+#include <cstdint>
+#include <string>
+#include <typeinfo>
+#include <vector>
+
+#include <any>
+#include <variant>
+
+#if __has_include(<optional>)
+    #include <optional>
+#elif __has_include(<experimental/optional>)
+#endif
+
+/////////////////////////////////////////////////////////////////////
+// for #4740
+/////////////////////////////////////////////////////////////////////
+
+struct Example_4740
+{
+    std::optional<std::string> host = std::nullopt;
+    std::optional<int> port = std::nullopt;
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(Example_4740, host, port)
+};
+
+TEST_CASE("regression tests 3 (C++17)")
+{
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+    SECTION("issue #3070 - Version 3.10.3 breaks backward-compatibility with 3.10.2 ")
+    {
+        nlohmann::detail::std_fs::path text_path("/tmp/text.txt");
+        const json j(text_path);
+
+        const auto j_path = j.get<nlohmann::detail::std_fs::path>();
+        CHECK(j_path == text_path);
+
+#if DOCTEST_CLANG || DOCTEST_GCC >= DOCTEST_COMPILER(8, 4, 0)
+        // only known to work on Clang and GCC >=8.4
+        CHECK_THROWS_WITH_AS(nlohmann::detail::std_fs::path(json(1)), "[json.exception.type_error.302] type must be string, but is number", json::type_error);
+#endif
+    }
+#endif
+
+#if JSON_USE_IMPLICIT_CONVERSIONS
+    SECTION("issue #3428 - Error occurred when converting nlohmann::json to std::any")
+    {
+        const json j;
+        const std::any a1 = j;
+        std::any&& a2 = j;
+
+        CHECK(a1.type() == typeid(j));
+        CHECK(a2.type() == typeid(j));
+    }
+#endif
+
+    SECTION("issue #4740 - build issue with std::optional")
+    {
+        const auto t1 = Example_4740();
+        const auto j1 = nlohmann::json(t1);
+        CHECK(j1.dump() == "{\"host\":null,\"port\":null}");
+        const auto t2 = j1.get<Example_4740>();
+        CHECK(!t2.host.has_value());
+        CHECK(!t2.port.has_value());
+
+        // improve coverage
+        auto t3 = Example_4740();
+        t3.port = 80;
+        t3.host = "example.com";
+        const auto j2 = nlohmann::json(t3);
+        CHECK(j2.dump() == "{\"host\":\"example.com\",\"port\":80}");
+        const auto t4 = j2.get<Example_4740>();
+        CHECK(t4.host.has_value());
+        CHECK(t4.port.has_value());
+    }
+
+    SECTION("issue #5046 - implicit conversion of return json to std::optional no longer implicit")
+    {
+        const json jval{};
+        auto GetValue = [](const json & valRoot) -> std::optional<json>
+        {
+            if (valRoot.contains("default"))
+            {
+                return valRoot.at("default");
+            }
+            return std::nullopt;
+        };
+        auto result = GetValue(jval);
+        CHECK(!result.has_value());
+    }
+
+}
+
+#endif

tests/src/unit-regression3-cpp20.cpp

@@ -0,0 +1,127 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++20-only part of unit-regression3.cpp (C++20 regression tests
+// (operator<=> related aggregates, ranges)). It is kept in a separate translation unit so
+// the (much larger) unit-regression3.cpp is built for C++11 only and not rebuilt for every
+// C++ standard.
+
+#include "doctest_compatibility.h"
+
+// skip tests if JSON_DisableEnumSerialization=ON (#4384): std::byte is a
+// scoped enum, so get<std::byte>() (needed below to get<std::vector<std::byte>>()
+// from a plain JSON array, not just from an already-binary value) relies on
+// enum serialization being enabled
+#if defined(JSON_DISABLE_ENUM_SERIALIZATION) && (JSON_DISABLE_ENUM_SERIALIZATION == 1)
+    #define SKIP_TESTS_FOR_ENUM_SERIALIZATION
+#endif
+
+#include <nlohmann/json.hpp>
+using json = nlohmann::json;
+using ordered_json = nlohmann::ordered_json;
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
+
+#ifdef JSON_HAS_CPP_20
+#include <string>
+
+#if __has_include(<span>)
+    #include <span>
+#endif
+
+/////////////////////////////////////////////////////////////////////
+// for #4440
+/////////////////////////////////////////////////////////////////////
+#if JSON_HAS_RANGES == 1
+    #include <ranges>
+#endif
+
+/////////////////////////////////////////////////////////////////////
+// for #3312
+/////////////////////////////////////////////////////////////////////
+
+struct for_3312
+{
+    std::string name;
+};
+
+inline void from_json(const json& j, for_3312& obj) // NOLINT(misc-use-internal-linkage)
+{
+    j.at("name").get_to(obj.name);
+}
+
+TEST_CASE("regression tests 3 (C++20)")
+{
+    SECTION("issue #3312 - Parse to custom class from unordered_json breaks on G++11.2.0 with C++20")
+    {
+        // see test for #3171
+        const ordered_json j = {{"name", "class"}};
+        for_3312 obj{};
+
+        j.get_to(obj);
+
+        CHECK(obj.name == "class");
+    }
+
+#if JSON_HAS_RANGES == 1
+    SECTION("issue #4440 - assert when using std::views::filter and GCC 10")
+    {
+        auto noOpFilter = std::views::filter([](auto&&) noexcept
+        {
+            return true;
+        });
+        json j = {1, 2, 3};
+        auto filtered = j | noOpFilter;
+        CHECK(*filtered.begin() == 1);
+    }
+#endif
+
+#if JSON_HAS_RANGE_VIEW_CONVERSION
+    SECTION("issue #4916 - constructing array from C++20 ranges view does not work")
+    {
+        std::vector<int> nums{1, 2, 37, 42, 21};
+        auto filteredNums = nums | std::views::filter([](int i)
+        {
+            return i > 10;
+        });
+        json const j(filteredNums);
+        CHECK(j.type() == json::value_t::array);
+        CHECK(j == json({37, 42, 21}));
+    }
+#endif
+
+    // owning_view is not available in libstdc++ < 12
+#if JSON_HAS_RANGE_VIEW_CONVERSION && !(defined(__GLIBCXX__) && _GLIBCXX_RELEASE < 12)
+    SECTION("issue #4916 - constructing array from prvalue C++20 ranges view (owning_view)")
+    {
+        json const j(std::vector<int> {1, 2, 37, 42, 21} | std::views::filter([](int i)
+        {
+            return i > 10;
+        }));
+        CHECK(j.type() == json::value_t::array);
+        CHECK(j == json({37, 42, 21}));
+    }
+#endif
+
+#if JSON_HAS_RANGE_VIEW_CONVERSION
+    SECTION("issue #4916 - constructing array from C++20 transform view (prvalue elements)")
+    {
+        std::vector<int> nums{1, 2, 3};
+        auto t = nums | std::views::transform([](int i) noexcept
+        {
+            return i * 2;
+        });
+        json const j(t);
+        CHECK(j.type() == json::value_t::array);
+        CHECK(j == json({2, 4, 6}));
+    }
+#endif
+}
+
+#endif

tests/src/unit-regression3.cpp

@@ -6,12 +6,11 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// When using macros that are only defined for particular versions of the standard
-// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
-// version macro in a comment close by, like this:
-// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. To avoid rebuilding this
+// large file for every standard, tests that depend on the standard version (e.g.,
+// those using JSON_HAS_FILESYSTEM, JSON_HAS_RANGES, or JSON_HAS_THREE_WAY_COMPARISON)
+// go into a separate file unit-regression3-cpp<NN>.cpp. This file stays C++11-only.
 
 #include "doctest_compatibility.h"
 
@@ -43,38 +42,12 @@ using ordered_json = nlohmann::ordered_json;
 #endif
 
 #include <cstdio>
-#include <deque>
 #include <list>
 #include <type_traits>
 #include <utility>
 
-#ifdef JSON_HAS_CPP_17
-    #include <any>
-    #include <variant>
-#endif
-
-#ifdef JSON_HAS_CPP_17
-    #if __has_include(<optional>)
-        #include <optional>
-    #elif __has_include(<experimental/optional>)
-    #endif
-#endif
-
-#ifdef JSON_HAS_CPP_20
-    #if __has_include(<span>)
-        #include <span>
-    #endif
-#endif
-
 // the explicit instantiation for #4825 is in unit-explicit_instantiation.cpp
 
-/////////////////////////////////////////////////////////////////////
-// for #4440
-/////////////////////////////////////////////////////////////////////
-#if JSON_HAS_RANGES == 1
-    #include <ranges>
-#endif
-
 // NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
 DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
@@ -189,22 +162,6 @@ inline void from_json(const json& j, for_3171_base& tb) // NOLINT(misc-use-inter
     tb._from_json(j);
 }
 
-/////////////////////////////////////////////////////////////////////
-// for #3312
-/////////////////////////////////////////////////////////////////////
-
-#ifdef JSON_HAS_CPP_20
-struct for_3312
-{
-    std::string name;
-};
-
-inline void from_json(const json& j, for_3312& obj) // NOLINT(misc-use-internal-linkage)
-{
-    j.at("name").get_to(obj.name);
-}
-#endif
-
 /////////////////////////////////////////////////////////////////////
 // for #3204
 /////////////////////////////////////////////////////////////////////
@@ -268,37 +225,8 @@ struct Example_3810
 
 NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Example_3810, bla) // NOLINT(misc-use-internal-linkage)
 
-/////////////////////////////////////////////////////////////////////
-// for #4740
-/////////////////////////////////////////////////////////////////////
-
-#ifdef JSON_HAS_CPP_17
-struct Example_4740
-{
-    std::optional<std::string> host = std::nullopt;
-    std::optional<int> port = std::nullopt;
-    NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(Example_4740, host, port)
-};
-#endif
-
 TEST_CASE("regression tests 3")
 {
-#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
-    // JSON_HAS_CPP_17 (do not remove; see note at top of file)
-    SECTION("issue #3070 - Version 3.10.3 breaks backward-compatibility with 3.10.2 ")
-    {
-        nlohmann::detail::std_fs::path text_path("/tmp/text.txt");
-        const json j(text_path);
-
-        const auto j_path = j.get<nlohmann::detail::std_fs::path>();
-        CHECK(j_path == text_path);
-
-#if DOCTEST_CLANG || DOCTEST_GCC >= DOCTEST_COMPILER(8, 4, 0)
-        // only known to work on Clang and GCC >=8.4
-        CHECK_THROWS_WITH_AS(nlohmann::detail::std_fs::path(json(1)), "[json.exception.type_error.302] type must be string, but is number", json::type_error);
-#endif
-    }
-#endif
 
     SECTION("issue #3077 - explicit constructor with default does not compile")
     {
@@ -349,31 +277,6 @@ TEST_CASE("regression tests 3")
         CHECK(td.str == "value");
     }
 
-#ifdef JSON_HAS_CPP_20
-    SECTION("issue #3312 - Parse to custom class from unordered_json breaks on G++11.2.0 with C++20")
-    {
-        // see test for #3171
-        const ordered_json j = {{"name", "class"}};
-        for_3312 obj{};
-
-        j.get_to(obj);
-
-        CHECK(obj.name == "class");
-    }
-#endif
-
-#if defined(JSON_HAS_CPP_17) && JSON_USE_IMPLICIT_CONVERSIONS
-    SECTION("issue #3428 - Error occurred when converting nlohmann::json to std::any")
-    {
-        const json j;
-        const std::any a1 = j;
-        std::any&& a2 = j;
-
-        CHECK(a1.type() == typeid(j));
-        CHECK(a2.type() == typeid(j));
-    }
-#endif
-
     SECTION("issue #3204 - ambiguous regression")
     {
         const for_3204_bar bar_from_foo([](for_3204_foo) noexcept {}); // NOLINT(performance-unnecessary-value-param)
@@ -416,28 +319,6 @@ TEST_CASE("regression tests 3")
         CHECK(oj["test"].dump() == expected);
     }
 
-#ifdef JSON_HAS_CPP_17
-    SECTION("issue #4740 - build issue with std::optional")
-    {
-        const auto t1 = Example_4740();
-        const auto j1 = nlohmann::json(t1);
-        CHECK(j1.dump() == "{\"host\":null,\"port\":null}");
-        const auto t2 = j1.get<Example_4740>();
-        CHECK(!t2.host.has_value());
-        CHECK(!t2.port.has_value());
-
-        // improve coverage
-        auto t3 = Example_4740();
-        t3.port = 80;
-        t3.host = "example.com";
-        const auto j2 = nlohmann::json(t3);
-        CHECK(j2.dump() == "{\"host\":\"example.com\",\"port\":80}");
-        const auto t4 = j2.get<Example_4740>();
-        CHECK(t4.host.has_value());
-        CHECK(t4.port.has_value());
-    }
-#endif
-
 #if !defined(_MSVC_LANG)
     // MSVC returns garbage on invalid enum values, so this test is excluded
     // there.
@@ -452,76 +333,7 @@ TEST_CASE("regression tests 3")
     }
 #endif
 
-#ifdef JSON_HAS_CPP_17
-    SECTION("issue #5046 - implicit conversion of return json to std::optional no longer implicit")
-    {
-        const json jval{};
-        auto GetValue = [](const json & valRoot) -> std::optional<json>
-        {
-            if (valRoot.contains("default"))
-            {
-                return valRoot.at("default");
-            }
-            return std::nullopt;
-        };
-        auto result = GetValue(jval);
-        CHECK(!result.has_value());
-    }
-#endif
-
-#if JSON_HAS_RANGES == 1
-    SECTION("issue #4440 - assert when using std::views::filter and GCC 10")
-    {
-        auto noOpFilter = std::views::filter([](auto&&) noexcept
-        {
-            return true;
-        });
-        json j = {1, 2, 3};
-        auto filtered = j | noOpFilter;
-        CHECK(*filtered.begin() == 1);
-    }
-#endif
-
-#if JSON_HAS_RANGE_VIEW_CONVERSION
-    SECTION("issue #4916 - constructing array from C++20 ranges view does not work")
-    {
-        std::vector<int> nums{1, 2, 37, 42, 21};
-        auto filteredNums = nums | std::views::filter([](int i)
-        {
-            return i > 10;
-        });
-        json const j(filteredNums);
-        CHECK(j.type() == json::value_t::array);
-        CHECK(j == json({37, 42, 21}));
-    }
-#endif
-
     // owning_view is not available in libstdc++ < 12
-#if JSON_HAS_RANGE_VIEW_CONVERSION && !(defined(__GLIBCXX__) && _GLIBCXX_RELEASE < 12)
-    SECTION("issue #4916 - constructing array from prvalue C++20 ranges view (owning_view)")
-    {
-        json const j(std::vector<int> {1, 2, 37, 42, 21} | std::views::filter([](int i)
-        {
-            return i > 10;
-        }));
-        CHECK(j.type() == json::value_t::array);
-        CHECK(j == json({37, 42, 21}));
-    }
-#endif
-
-#if JSON_HAS_RANGE_VIEW_CONVERSION
-    SECTION("issue #4916 - constructing array from C++20 transform view (prvalue elements)")
-    {
-        std::vector<int> nums{1, 2, 3};
-        auto t = nums | std::views::transform([](int i) noexcept
-        {
-            return i * 2;
-        });
-        json const j(t);
-        CHECK(j.type() == json::value_t::array);
-        CHECK(j == json({2, 4, 6}));
-    }
-#endif
 }
 
 TEST_CASE_TEMPLATE("issue #4798 - nlohmann::json::to_msgpack() encode float NaN as double", T, double, float) // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
@@ -867,52 +679,6 @@ TEST_CASE("issue #5402 - update(merge_objects=true) overwrites a primitive with
 }
 
 
-TEST_CASE("regression test #5476 - array type without reserve()")
-{
-    // the capacity reserved for definite-length arrays must not require the
-    // array type to have a reserve() member function
-    using deque_json = nlohmann::json::with_array_t<std::deque>;
-
-    SECTION("std::deque")
-    {
-        const auto j = deque_json::parse(R"({"a":[1,[2,3]],"b":[]})");
-        CHECK(j.dump() == R"({"a":[1,[2,3]],"b":[]})");
-
-        // the binary formats pass a definite length to start_array()
-        CHECK(deque_json::from_cbor(deque_json::to_cbor(j)) == j);
-        CHECK(deque_json::from_msgpack(deque_json::to_msgpack(j)) == j);
-        CHECK(deque_json::from_bon8(deque_json::to_bon8(j)) == j);
-
-        // parse() instantiates the callback parser as well, which reserves too
-        const auto with_callback = deque_json::parse(R"([1,2,3])", [](int /*depth*/, deque_json::parse_event_t /*event*/, deque_json& /*parsed*/) noexcept
-        {
-            return true;
-        });
-        CHECK(with_callback == deque_json({1, 2, 3}));
-    }
-
-    SECTION("std::vector still reserves")
-    {
-        json array = json::array();
-        for (int i = 0; i < 100; ++i)
-        {
-            array.push_back(i);
-        }
-
-        const auto j = json::from_cbor(json::to_cbor(array));
-        CHECK(j == array);
-        CHECK(j.get_ref<const json::array_t&>().capacity() >= 100);
-    }
-
-    SECTION("the reservation stays capped")
-    {
-        // CBOR array announcing 2^32-1 elements, but truncated right after the
-        // header: the input must be rejected without reserving that capacity
-        const std::vector<std::uint8_t> truncated = {0x9A, 0xFF, 0xFF, 0xFF, 0xFF};
-        CHECK(json::from_cbor(truncated, true, false).is_discarded());
-    }
-}
-
 TEST_CASE("issue #5317 - nested indefinite-length CBOR string chunks are rejected")
 {
     json _;

tests/src/unit-std-format.cpp

@@ -6,12 +6,10 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// When using macros that are only defined for particular versions of the standard
-// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
-// version macro in a comment close by, like this:
-// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. This whole file tests
+// std::format (C++20), so it cannot be split: it is built for C++20 as well:
+// JSON_HAS_CPP_20 (do not remove)
 
 #include "doctest_compatibility.h"
 
@@ -19,7 +17,6 @@
 using json = nlohmann::json;
 using ordered_json = nlohmann::ordered_json;
 
-// JSON_HAS_CPP_20 (do not remove; see note at top of file)
 #if JSON_HAS_STD_FORMAT
 
 #include <iterator>

tests/src/unit-to_chars.cpp

@@ -846,3 +846,85 @@ TEST_CASE("choice of the conversion")
         DOCTEST_MSVC_SUPPRESS_WARNING_POP
     }
 }
+
+TEST_CASE("short decimals")
+{
+    // write_short_decimal() writes digits * 10^exp for the digits of a double
+    // that need no conversion (at most 15, the first not 0): as to_chars()
+    // writes the (positive) double that has these digits
+    const auto written = [](std::uint64_t digits, int exp) -> std::string
+    {
+        std::array<char, 64> buf{}; // (up to 41 bytes are written)
+        char* const end = nlohmann::detail::dtoa_impl::write_short_decimal(buf.data(), digits, exp);
+        return {buf.data(), end};
+    };
+    const auto written_counted = [](std::uint64_t digits, int count, int exp) -> std::string
+    {
+        std::array<char, 64> buf{};
+        char* const end = nlohmann::detail::dtoa_impl::write_short_decimal(buf.data(), digits, count, exp);
+        return {buf.data(), end};
+    };
+    const auto expected = [](std::uint64_t digits, int exp) -> std::string
+    {
+        const double value = std::strtod((std::to_string(digits) + "e" + std::to_string(exp)).c_str(), nullptr);
+        std::array<char, 64> buf{};
+        char* const end = nlohmann::detail::to_chars(buf.data(), buf.data() + 32, value);
+        return {buf.data(), end};
+    };
+
+    SECTION("powers of ten")
+    {
+        const auto& powers = nlohmann::detail::dtoa_impl::powers_of_ten();
+        CHECK(powers.front() == 1u);
+        for (std::size_t i = 1; i < powers.size(); ++i)
+        {
+            CHECK(powers[i] == powers[i - 1] * 10u);
+        }
+        CHECK(powers.back() == 10000000000000000000u);
+    }
+
+    SECTION("examples")
+    {
+        CHECK(written(1, 0) == "1.0");
+        CHECK(written(15, -1) == "1.5");
+        CHECK(written(125, -2) == "1.25");
+        CHECK(written(1, 22) == "1e+22");
+        CHECK(written(123456789012345, -2) == "1234567890123.45");
+        CHECK(written(999999999999999, -15) == "0.999999999999999");
+        CHECK(written(5, -324) == "5e-324");
+        CHECK(written_counted(1, 1, 0) == "1.0");
+        CHECK(written_counted(125, 3, -2) == "1.25");
+        CHECK(written_counted(100, 3, -2) == "1.0");
+        CHECK(written_counted(999999999999999, 15, -15) == "0.999999999999999");
+    }
+
+    SECTION("random digits, exponents and trailing zeros")
+    {
+        std::mt19937_64 rng(1170); // NOLINT(cert-msc32-c,cert-msc51-cpp,bugprone-random-generator-seed)
+        for (int i = 0; i < 100000; ++i)
+        {
+            // 1 to 15 digits, the first not 0, and up to 14 of them trailing zeros
+            std::uint64_t count = 1 + (rng() % 15u);
+            std::uint64_t power = 1;
+            for (std::uint64_t k = 1; k < count; ++k)
+            {
+                power *= 10;
+            }
+            std::uint64_t digits = power + (rng() % (9 * power));
+            const std::uint64_t zeros = (rng() % 3u == 0) ? (rng() % count) : 0;
+            for (std::uint64_t k = 0; k < zeros; ++k)
+            {
+                digits = (digits / 10) * 10;
+            }
+            // (a value between 1e-300 and 1e300)
+            const int exp = static_cast<int>(rng() % 560u) - 300 - static_cast<int>(count);
+
+            CAPTURE(digits)
+            CAPTURE(count)
+            CAPTURE(exp)
+            const std::string want = expected(digits, exp);
+            CHECK(written(digits, exp) == want);
+            CHECK(written_counted(digits, static_cast<int>(count), exp) == want);
+        }
+    }
+}

tests/src/unit-user_defined_input-cpp20.cpp

@@ -0,0 +1,219 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// This file contains the C++20-only part of unit-user_defined_input.cpp (parsing with
+// std::counted_iterator and std::default_sentinel_t). It is kept in a separate translation
+// unit so the (much larger) unit-user_defined_input.cpp is built for C++11 only and not
+// rebuilt for every C++ standard.
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+#ifdef JSON_HAS_CPP_20
+#include <cstddef>
+#include <string>
+#include <vector>
+
+#if defined(__cpp_lib_concepts)
+    #include <iterator>
+#endif
+
+#if defined(__cpp_lib_concepts)
+TEST_CASE("Parse with std::counted_iterator and std::default_sentinel_t")
+{
+    using iterator_type = std::string::const_iterator;
+    const std::string json_str = R"({"key":"value","array":[1,2,3]})";
+    const auto len = static_cast<std::iter_difference_t<iterator_type>>(json_str.size());
+
+    const std::counted_iterator<iterator_type> first(json_str.begin(), len);
+    const json j = json::parse(first, std::default_sentinel);
+    CHECK(j["key"] == "value");
+    CHECK(j["array"].size() == 3);
+
+    const std::counted_iterator<iterator_type> first2(json_str.begin(), len);
+    CHECK(json::accept(first2, std::default_sentinel));
+}
+
+TEST_CASE("std::counted_iterator reaches the contiguous fast paths")
+{
+    // A sized sentinel makes the remaining element count computable in O(1), so
+    // std::counted_iterator over a contiguous iterator must reach the same bulk
+    // string/number scanners as a plain pointer - not just the byte-at-a-time
+    // fallback (see #5268 for the equivalent memcpy fast path).
+#if JSON_HAS_RANGES
+    // JSON_HAS_RANGES is 0 on standard libraries with an incomplete <ranges>
+    // (libstdc++ < 11, libc++ < 16), where the adapter deliberately falls back
+    // to the byte-at-a-time scanner; everything below still has to work there.
+    using adapter_type = nlohmann::detail::iterator_input_adapter<std::counted_iterator<const char*>, std::default_sentinel_t>;
+    CHECK(adapter_type::supports_bulk_scan);
+    CHECK(adapter_type::supports_seek);
+#endif
+
+    // exercise every fast path: long ASCII run, multibyte UTF-8, escapes, and
+    // integer/floating-point numbers
+    const std::string json_str =
+        R"({"ascii":"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",)"
+        "\"utf8\":\"\xe4\xb8\xad\xe6\x96\x87\xf0\x9f\x98\x80\xc3\xa9\","
+        R"("escaped":"aéb\n\\","ints":[0,-1,18446744073709551615,-9223372036854775808],)"
+        R"("floats":[1.5,-2.25e3,0.30000000000000004]})";
+    const auto len = static_cast<std::iter_difference_t<const char*>>(json_str.size());
+
+    const std::counted_iterator<const char*> first(json_str.data(), len);
+    const json j = json::parse(first, std::default_sentinel);
+
+    // parsing through the pointer adapter must give exactly the same result
+    CHECK(j == json::parse(json_str));
+
+#if !defined(JSON_NOEXCEPTION)
+    // Diagnostics that quote the offending token are reconstructed from the
+    // already-consumed input (supports_seek), a path a sized sentinel only
+    // reaches now; check a few that include the "last read" text. Parsing
+    // invalid input aborts when exceptions are off, hence the guard.
+    // Raw strings and explicit bytes: an escaped literal and two literals
+    // written next to each other both read as mistakes to static analysis.
+    const auto byte = [](int value)
+    {
+        return std::string(1, static_cast<char>(value));
+    };
+    const std::vector<std::string> diagnostic_docs =
+    {
+        "1\nx",
+        "truX",
+        "[tru]",
+        R"("abc)",
+        R"(["\ud834"])",
+        R"(["a)" + byte(0x01) + R"(b"])",
+        R"([")" + byte(0xC3) + byte(0x28) + R"("])",
+        "[1e]",
+        R"(["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaX)"
+    };
+
+    for (const auto& text : diagnostic_docs)
+    {
+        CAPTURE(text)
+        const std::counted_iterator<const char*> it(text.data(), static_cast<std::iter_difference_t<const char*>>(text.size()));
+        std::string counted_message;
+        std::string string_message;
+        try
+        {
+            const json counted_result = json::parse(it, std::default_sentinel);
+            static_cast<void>(counted_result);
+        }
+        catch (const json::parse_error& e)
+        {
+            counted_message = e.what();
+        }
+        try
+        {
+            const json string_result = json::parse(text);
+            static_cast<void>(string_result);
+        }
+        catch (const json::parse_error& e)
+        {
+            string_message = e.what();
+        }
+        CHECK_FALSE(counted_message.empty());
+        CHECK(counted_message == string_message);
+    }
+
+    // and errors must still be reported identically
+    const std::string bad = "[01\n]";
+    const std::counted_iterator<const char*> bad_first(bad.data(), static_cast<std::iter_difference_t<const char*>>(bad.size()));
+    std::string counted_what;
+    std::string string_what;
+    try
+    {
+        const json counted_result = json::parse(bad_first, std::default_sentinel);
+        static_cast<void>(counted_result);
+    }
+    catch (const json::parse_error& e)
+    {
+        counted_what = e.what();
+    }
+    try
+    {
+        const json string_result = json::parse(bad);
+        static_cast<void>(string_result);
+    }
+    catch (const json::parse_error& e)
+    {
+        string_what = e.what();
+    }
+    CHECK_FALSE(counted_what.empty());
+    CHECK(counted_what == string_what);
+#endif
+}
+
+#if !defined(JSON_NOEXCEPTION)
+// several cases below are truncated on purpose, and parsing invalid input
+// aborts when exceptions are off
+TEST_CASE("std::counted_iterator bulk scanning stops at the counted end")
+{
+    // The count, not the size of the underlying buffer, is the end of the
+    // input: the bulk scanners must never look at the bytes behind it, even
+    // though they are readable. Each case is compared against parsing the
+    // equivalent prefix as a std::string.
+    const auto via_counted = [](const std::string & buf, std::size_t n) -> std::string
+    {
+        const std::counted_iterator<const char*> first(buf.data(), static_cast<std::iter_difference_t<const char*>>(n));
+        try
+        {
+            const json j = json::parse(first, std::default_sentinel);
+            return "OK|" + j.dump();
+        }
+        catch (const json::parse_error& e)
+        {
+            return {e.what()};
+        }
+    };
+    const auto via_prefix = [](const std::string & buf, std::size_t n) -> std::string
+    {
+        try
+        {
+            const json j = json::parse(buf.substr(0, n));
+            return "OK|" + j.dump();
+        }
+        catch (const json::parse_error& e)
+        {
+            return {e.what()};
+        }
+    };
+
+    struct testcase // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init)
+    {
+        const char* buffer;
+        std::size_t count;
+    };
+    const std::vector<testcase> cases =
+    {
+        {"[\"abc\"]____TRAILING____", 7},                    // exact fit, tail hidden
+        {"[\"abcdefghijklmnop\"]____", 8},                   // cut inside a string
+        {"[\"abc\"]____", 6},                                // cut just before the closing quote
+        {"[12345]xxxxx", 4},                                 // cut inside a number
+        {"[123]999999", 5},                                  // number ends exactly at the count
+        {"[\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\"]", 12},        // closing quote only behind the count
+        {"[\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\"]", 19},      // cut inside an 8-byte SWAR stride
+        {"[\"\xe4\xb8\xad\xe6\x96\x87\"]", 5},               // cut inside a UTF-8 sequence
+        {"[\"\xe4\xb8\xad\xe6\x96\x87\"]____", 10},          // complete UTF-8, tail hidden
+        {"[1.25e3]TRAILINGDIGITS999", 7},                    // number token reaches the count
+    };
+
+    for (const auto& tc : cases)
+    {
+        CAPTURE(tc.buffer)
+        CAPTURE(tc.count)
+        const std::string buffer = tc.buffer;
+        CHECK(via_counted(buffer, tc.count) == via_prefix(buffer, tc.count));
+    }
+}
+#endif
+#endif
+
+#endif

tests/src/unit-user_defined_input.cpp

@@ -6,12 +6,11 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-// cmake/test.cmake selects the C++ standard versions with which to build a
-// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
-// When using macros that are only defined for particular versions of the standard
-// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
-// version macro in a comment close by, like this:
-// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+// cmake/test.cmake builds a unit test with C++ standards beyond C++11 only if the
+// source file mentions the corresponding version macro. To avoid rebuilding this
+// large file for every standard, tests that depend on the standard version (e.g.,
+// those using JSON_HAS_FILESYSTEM, JSON_HAS_RANGES, or JSON_HAS_THREE_WAY_COMPARISON)
+// go into a separate file unit-user_defined_input-cpp<NN>.cpp. This file stays C++11-only.
 
 #include "doctest_compatibility.h"
 
@@ -25,10 +24,6 @@ using nlohmann::json;
 #include <string> // string
 #include <vector> // vector
 
-#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
-    #include <iterator>
-#endif
-
 namespace
 {
 TEST_CASE("Use arbitrary stdlib container")
@@ -277,196 +272,4 @@ TEST_CASE("Contiguous byte containers take the pointer adapter")
     CHECK(json::parse(buffer) == json({1, 2, 3, 4, 5}));
 }
 
-#if defined(__cpp_lib_concepts) && defined(JSON_HAS_CPP_20)
-// JSON_HAS_CPP_20 (do not remove; see note at top of file)
-TEST_CASE("Parse with std::counted_iterator and std::default_sentinel_t")
-{
-    using iterator_type = std::string::const_iterator;
-    const std::string json_str = R"({"key":"value","array":[1,2,3]})";
-    const auto len = static_cast<std::iter_difference_t<iterator_type>>(json_str.size());
-
-    const std::counted_iterator<iterator_type> first(json_str.begin(), len);
-    const json j = json::parse(first, std::default_sentinel);
-    CHECK(j["key"] == "value");
-    CHECK(j["array"].size() == 3);
-
-    const std::counted_iterator<iterator_type> first2(json_str.begin(), len);
-    CHECK(json::accept(first2, std::default_sentinel));
-}
-
-TEST_CASE("std::counted_iterator reaches the contiguous fast paths")
-{
-    // A sized sentinel makes the remaining element count computable in O(1), so
-    // std::counted_iterator over a contiguous iterator must reach the same bulk
-    // string/number scanners as a plain pointer - not just the byte-at-a-time
-    // fallback (see #5268 for the equivalent memcpy fast path).
-#if JSON_HAS_RANGES
-    // JSON_HAS_RANGES is 0 on standard libraries with an incomplete <ranges>
-    // (libstdc++ < 11, libc++ < 16), where the adapter deliberately falls back
-    // to the byte-at-a-time scanner; everything below still has to work there.
-    using adapter_type = nlohmann::detail::iterator_input_adapter<std::counted_iterator<const char*>, std::default_sentinel_t>;
-    CHECK(adapter_type::supports_bulk_scan);
-    CHECK(adapter_type::supports_seek);
-#endif
-
-    // exercise every fast path: long ASCII run, multibyte UTF-8, escapes, and
-    // integer/floating-point numbers
-    const std::string json_str =
-        R"({"ascii":"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",)"
-        "\"utf8\":\"\xe4\xb8\xad\xe6\x96\x87\xf0\x9f\x98\x80\xc3\xa9\","
-        R"("escaped":"aéb\n\\","ints":[0,-1,18446744073709551615,-9223372036854775808],)"
-        R"("floats":[1.5,-2.25e3,0.30000000000000004]})";
-    const auto len = static_cast<std::iter_difference_t<const char*>>(json_str.size());
-
-    const std::counted_iterator<const char*> first(json_str.data(), len);
-    const json j = json::parse(first, std::default_sentinel);
-
-    // parsing through the pointer adapter must give exactly the same result
-    CHECK(j == json::parse(json_str));
-
-#if !defined(JSON_NOEXCEPTION)
-    // Diagnostics that quote the offending token are reconstructed from the
-    // already-consumed input (supports_seek), a path a sized sentinel only
-    // reaches now; check a few that include the "last read" text. Parsing
-    // invalid input aborts when exceptions are off, hence the guard.
-    // Raw strings and explicit bytes: an escaped literal and two literals
-    // written next to each other both read as mistakes to static analysis.
-    const auto byte = [](int value)
-    {
-        return std::string(1, static_cast<char>(value));
-    };
-    const std::vector<std::string> diagnostic_docs =
-    {
-        "1\nx",
-        "truX",
-        "[tru]",
-        R"("abc)",
-        R"(["\ud834"])",
-        R"(["a)" + byte(0x01) + R"(b"])",
-        R"([")" + byte(0xC3) + byte(0x28) + R"("])",
-        "[1e]",
-        R"(["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaX)"
-    };
-
-    for (const auto& text : diagnostic_docs)
-    {
-        CAPTURE(text)
-        const std::counted_iterator<const char*> it(text.data(), static_cast<std::iter_difference_t<const char*>>(text.size()));
-        std::string counted_message;
-        std::string string_message;
-        try
-        {
-            const json counted_result = json::parse(it, std::default_sentinel);
-            static_cast<void>(counted_result);
-        }
-        catch (const json::parse_error& e)
-        {
-            counted_message = e.what();
-        }
-        try
-        {
-            const json string_result = json::parse(text);
-            static_cast<void>(string_result);
-        }
-        catch (const json::parse_error& e)
-        {
-            string_message = e.what();
-        }
-        CHECK_FALSE(counted_message.empty());
-        CHECK(counted_message == string_message);
-    }
-
-    // and errors must still be reported identically
-    const std::string bad = "[01\n]";
-    const std::counted_iterator<const char*> bad_first(bad.data(), static_cast<std::iter_difference_t<const char*>>(bad.size()));
-    std::string counted_what;
-    std::string string_what;
-    try
-    {
-        const json counted_result = json::parse(bad_first, std::default_sentinel);
-        static_cast<void>(counted_result);
-    }
-    catch (const json::parse_error& e)
-    {
-        counted_what = e.what();
-    }
-    try
-    {
-        const json string_result = json::parse(bad);
-        static_cast<void>(string_result);
-    }
-    catch (const json::parse_error& e)
-    {
-        string_what = e.what();
-    }
-    CHECK_FALSE(counted_what.empty());
-    CHECK(counted_what == string_what);
-#endif
-}
-
-#if !defined(JSON_NOEXCEPTION)
-// several cases below are truncated on purpose, and parsing invalid input
-// aborts when exceptions are off
-TEST_CASE("std::counted_iterator bulk scanning stops at the counted end")
-{
-    // The count, not the size of the underlying buffer, is the end of the
-    // input: the bulk scanners must never look at the bytes behind it, even
-    // though they are readable. Each case is compared against parsing the
-    // equivalent prefix as a std::string.
-    const auto via_counted = [](const std::string & buf, std::size_t n) -> std::string
-    {
-        const std::counted_iterator<const char*> first(buf.data(), static_cast<std::iter_difference_t<const char*>>(n));
-        try
-        {
-            const json j = json::parse(first, std::default_sentinel);
-            return "OK|" + j.dump();
-        }
-        catch (const json::parse_error& e)
-        {
-            return {e.what()};
-        }
-    };
-    const auto via_prefix = [](const std::string & buf, std::size_t n) -> std::string
-    {
-        try
-        {
-            const json j = json::parse(buf.substr(0, n));
-            return "OK|" + j.dump();
-        }
-        catch (const json::parse_error& e)
-        {
-            return {e.what()};
-        }
-    };
-
-    struct testcase // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init)
-    {
-        const char* buffer;
-        std::size_t count;
-    };
-    const std::vector<testcase> cases =
-    {
-        {"[\"abc\"]____TRAILING____", 7},                    // exact fit, tail hidden
-        {"[\"abcdefghijklmnop\"]____", 8},                   // cut inside a string
-        {"[\"abc\"]____", 6},                                // cut just before the closing quote
-        {"[12345]xxxxx", 4},                                 // cut inside a number
-        {"[123]999999", 5},                                  // number ends exactly at the count
-        {"[\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\"]", 12},        // closing quote only behind the count
-        {"[\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\"]", 19},      // cut inside an 8-byte SWAR stride
-        {"[\"\xe4\xb8\xad\xe6\x96\x87\"]", 5},               // cut inside a UTF-8 sequence
-        {"[\"\xe4\xb8\xad\xe6\x96\x87\"]____", 10},          // complete UTF-8, tail hidden
-        {"[1.25e3]TRAILINGDIGITS999", 7},                    // number token reaches the count
-    };
-
-    for (const auto& tc : cases)
-    {
-        CAPTURE(tc.buffer)
-        CAPTURE(tc.count)
-        const std::string buffer = tc.buffer;
-        CHECK(via_counted(buffer, tc.count) == via_prefix(buffer, tc.count));
-    }
-}
-#endif
-#endif
-
 } // namespace