Document what counts as a breaking change in the API stability rules

Spell out the 3.x compatibility rules in the roadmap: new defaulted
parameters, new default arguments, noexcept/constexpr, template
parameters, parse and dump results, accepted input, key iteration order,
iterator invalidation, implicit conversions, to_json/from_json lookup,
json_sax, value_t enumerators, and documented macros, CMake options and
headers. Also list std::hash values as not part of the public API, and
link the macro overview from the section.

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

.github/workflows/ubuntu.yml

@@ -31,6 +31,51 @@ jobs:
       - name: Build
         run: cmake --build build --target ci_test_gcc
 
+  ci_meson_install:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@fffaaafeea488556c2c12dad60690008bc1caacb # v4.4.2
+      - name: Check that Meson and pkg-config offer the CMake options
+        run: make check_build_options
+      - name: Install Meson
+        run: pip install meson
+      - name: Install with Meson
+        run: |
+          meson setup build-meson --prefix=${{ github.workspace }}/install
+          meson install -C build-meson
+      - name: Use the installed package with find_package
+        run: |
+          cmake -S tests/cmake_import/project -B build-import -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install
+          cmake --build build-import
+      - name: Install with Meson and non-default options
+        run: |
+          meson setup build-meson-options --prefix=${{ github.workspace }}/install-options -DMultipleHeaders=true -DDiagnostics=true -DGlobalUDLs=false -DDisableTupleReferenceConversion=true -DStrictBinaryUTF8=true -DDeleteDeprecatedFunctions=true
+          meson install -C build-meson-options
+      - name: Check that the options reach the installed files
+        run: |
+          test -d install-options/include/nlohmann/detail
+          cflags=$(PKG_CONFIG_PATH=${{ github.workspace }}/install-options/share/pkgconfig pkg-config --cflags nlohmann_json)
+          echo "$cflags"
+          echo "$cflags" | grep -q -- '-DJSON_DIAGNOSTICS=1'
+          echo "$cflags" | grep -q -- '-DJSON_USE_GLOBAL_UDLS=0'
+          echo "$cflags" | grep -q -- '-DJSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1'
+          echo "$cflags" | grep -q -- '-DJSON_STRICT_BINARY_UTF8=1'
+          echo "$cflags" | grep -q -- '-DJSON_DELETE_DEPRECATED_FUNCTIONS=1'
+          grep -q 'JSON_USE_GLOBAL_UDLS=0;JSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1;JSON_DIAGNOSTICS=1' install-options/share/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
+          grep -q 'JSON_STRICT_BINARY_UTF8=1;JSON_DELETE_DEPRECATED_FUNCTIONS=1' install-options/share/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
+          cmake -S tests/cmake_import/project -B build-import-options -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-options
+          cmake --build build-import-options
+      - name: Install with Meson and the include directory outside the prefix
+        run: |
+          meson setup build-meson-split --prefix=${{ github.workspace }}/install-split --includedir=${{ github.workspace }}/install-split-dev/include
+          meson install -C build-meson-split
+          cmake -S tests/cmake_import/project -B build-import-split -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-split
+          cmake --build build-import-split
+
   ci_infer:
     runs-on: ubuntu-latest
     steps:

CMakeLists.txt

@@ -189,7 +189,39 @@ if (MSVC)
     )
 endif()
 
-# Install a pkg-config file, so other tools can find this.
+# Install a pkg-config file, so other tools can find this. It carries the same
+# compile definitions as the target above.
+set(NLOHMANN_JSON_PKGCONFIG_CFLAGS "")
+if (NOT JSON_GlobalUDLs)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_GLOBAL_UDLS=0")
+endif()
+if (NOT JSON_ImplicitConversions)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_IMPLICIT_CONVERSIONS=0")
+endif()
+if (JSON_DisableEnumSerialization)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DISABLE_ENUM_SERIALIZATION=1")
+endif()
+if (JSON_DisableTupleReferenceConversion)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1")
+endif()
+if (JSON_Diagnostics)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DIAGNOSTICS=1")
+endif()
+if (JSON_Diagnostic_Positions)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DIAGNOSTIC_POSITIONS=1")
+endif()
+if (JSON_LegacyDiscardedValueComparison)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1")
+endif()
+if (JSON_StrictNulHandling)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_STRICT_NUL_HANDLING=1")
+endif()
+if (JSON_StrictBinaryUTF8)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_STRICT_BINARY_UTF8=1")
+endif()
+if (JSON_DeleteDeprecatedFunctions)
+    string(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -DJSON_DELETE_DEPRECATED_FUNCTIONS=1")
+endif()
 configure_file(
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake/pkg-config.pc.in"
     "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"

FILES.md

@@ -249,9 +249,31 @@ make BUILD.bazel
 
 The "Check amalgamation" workflow fails if the file is out of date.
 
-### `meson.build`
+### `meson.build` and `meson_options.txt`
 
-The build definition for the [Meson](https://mesonbuild.com) build system.
+Meson build definitions suitable for use as a subproject ("wrap" in Meson terminology).
+
+Projects wishing to use the wrap can execute:
+```sh
+meson wrap install nlohmann_json
+```
+
+Which allows Meson to build from source when a system provided dependency isn't available.
+
+To build directly:
+```sh
+meson setup builddir
+ninja -C builddir
+```
+
+`meson_options.txt` defines the options, which mirror the CMake options that change the library's target (for example,
+`-DDiagnostics=true`). Meson requires this file next to `meson.build`, so it is also part of `include.zip`. `make check_build_options`
+([`tools/check_build_options`](tools/check_build_options/README.md)) checks in CI that both files and the pkg-config files
+stay in sync with the CMake options.
+
+When installing, `meson.build` installs the headers, a pkg-config file, and the CMake package config files, so that
+`find_package(nlohmann_json)` works. As Meson cannot generate `nlohmann_jsonTargets.cmake` itself, it is created from
+the template `cmake/nlohmann_jsonTargets.cmake.in`, which is only used by Meson.
 
 ### `Package.swift`
 

Makefile

@@ -1,4 +1,4 @@
-.PHONY: pretty clean ChangeLog.md release update_hedley update_hedley_undef BUILD.bazel natvis macro_builder_check
+.PHONY: pretty clean ChangeLog.md release update_hedley update_hedley_undef BUILD.bazel natvis macro_builder_check check_build_options
 
 ##########################################################################
 # configuration
@@ -127,6 +127,10 @@ macro_builder_check:
 	diff "$$TMPDIR/paste.hpp" "$$TMPDIR/paste_actual.hpp" || (echo "===================================================================\n  $(MACRO_SCOPE_HPP) (NLOHMANN_JSON_EXPAND..NLOHMANN_JSON_DOUBLE_PASTE63) is out of date!\n  Regenerate it, see tools/macro_builder/README.md.\n===================================================================" ; exit 1); \
 	diff "$$TMPDIR/type_body.hpp" "$$TMPDIR/type_body_actual.hpp" || (echo "===================================================================\n  $(MACRO_SCOPE_HPP) (NLOHMANN_JSON_TYPE_BODY) is out of date!\n  Regenerate it, see tools/macro_builder/README.md.\n===================================================================" ; exit 1)
 
+# check that the Meson build and the pkg-config files offer the options of the CMake target
+check_build_options:
+	python3 tools/check_build_options/check_build_options.py .
+
 # check if file single_include/nlohmann/json.hpp has been amalgamated from the nlohmann sources
 check-amalgamation:
 	@mv $(AMALGAMATED_FILE) $(AMALGAMATED_FILE)~
@@ -184,7 +188,7 @@ json.tar.xz:
 # We use `-X` to make the resulting ZIP file reproducible, see
 # <https://content.pivotal.io/blog/barriers-to-deterministic-reproducible-zip-files>.
 include.zip: BUILD.bazel
-	zip -9 --recurse-paths -X include.zip $(SRCS) $(AMALGAMATED_FILE) $(AMALGAMATED_FWD_FILE) $(AMALGAMATED_LITERALS_FILE) BUILD.bazel MODULE.bazel meson.build LICENSE.MIT
+	zip -9 --recurse-paths -X include.zip $(SRCS) $(AMALGAMATED_FILE) $(AMALGAMATED_FWD_FILE) $(AMALGAMATED_LITERALS_FILE) BUILD.bazel MODULE.bazel meson.build meson_options.txt LICENSE.MIT
 
 # Create the files for a release and add signatures and hashes.
 release: include.zip json.tar.xz

cmake/nlohmann_jsonTargets.cmake.in

@@ -0,0 +1,42 @@
+# Imported target for installations made with Meson (see meson.build).
+#
+# CMake installations generate this file with install(EXPORT ...). Meson cannot
+# do that, but as the library is header-only, the target only needs an include
+# directory, the C++ standard, and the compile definitions of the options that
+# differ from their defaults. Paths are computed relative to this file so that
+# the installation can be relocated (e.g., into a sysroot), unless includedir or
+# datadir is outside the prefix.
+
+if(TARGET @PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@)
+    return()
+endif()
+
+get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_DIR}/@NLOHMANN_JSON_CONFIG_TO_PREFIX@" ABSOLUTE)
+# As in CMake's generated file: avoid "//include" for an installation to "/".
+if(_IMPORT_PREFIX STREQUAL "/")
+    set(_IMPORT_PREFIX "")
+endif()
+
+add_library(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ INTERFACE IMPORTED)
+set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES "@NLOHMANN_JSON_INCLUDE_DIR@"
+)
+if(CMAKE_VERSION VERSION_LESS 3.8)
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_FEATURES cxx_range_for
+    )
+else()
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_FEATURES cxx_std_11
+    )
+endif()
+
+set(_NLOHMANN_JSON_COMPILE_DEFINITIONS "@NLOHMANN_JSON_COMPILE_DEFINITIONS@")
+if(_NLOHMANN_JSON_COMPILE_DEFINITIONS)
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_DEFINITIONS "${_NLOHMANN_JSON_COMPILE_DEFINITIONS}"
+    )
+endif()
+
+unset(_NLOHMANN_JSON_COMPILE_DEFINITIONS)
+unset(_IMPORT_PREFIX)

cmake/pkg-config.pc.in

@@ -4,4 +4,4 @@ includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
 Name: @PROJECT_NAME@
 Description: JSON for Modern C++
 Version: @PROJECT_VERSION@
-Cflags: -I${includedir}
+Cflags: -I${includedir}@NLOHMANN_JSON_PKGCONFIG_CFLAGS@

docs/mkdocs/docs/api/basic_json/operator[].md

@@ -137,13 +137,10 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
         When the JSON pointer traverses intermediate levels that don't exist at all yet (not just a missing
         leaf), each missing level is created as an array or an object depending on whether the corresponding
-        pointer token is a valid array index: the token `0`, a sequence of digits that does not begin with `0`,
-        or the token `-` creates an array, and every other token creates an object. For example, on an
-        initially `#!json null` value, `/foo/0/0/0` creates nested arrays, while `/foo/one/one/one` creates
-        nested objects. Tokens such as `01` or the empty token cannot be array indices (cf. RFC 6901, Sect. 4)
-        and therefore create objects, just as they would if the level already existed as an object. This is not
-        specified by the JSON Pointer RFC; it is this library's own, intentional disambiguation rule. See also
-        [JSON Pointer](../../features/json_pointer.md).
+        pointer token parses as a non-negative integer: a numeric token creates an array, a non-numeric token
+        creates an object. For example, on an initially `#!json null` value, `/foo/0/0/0` creates nested arrays,
+        while `/foo/one/one/one` creates nested objects. This is not specified by the JSON Pointer RFC; it is
+        this library's own, intentional disambiguation rule. See also [JSON Pointer](../../features/json_pointer.md).
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/community/roadmap.md

@@ -36,13 +36,26 @@ work items are tracked in the [GitHub milestones](https://github.com/nlohmann/js
 ## API stability
 
 Releases follow [semantic versioning](https://semver.org): a minor or patch release of version 3.x does not break code
-that uses the public API. In particular, a 3.x release does not:
+that uses the public API, unless that code opts in to a change with a macro as described [below](#version-40). In
+particular, a 3.x release does not:
 
-- change the signature of a function (its parameter types, return type, number of parameters, or the const-ness of a
-  member function);
-- remove or rename a function or class;
+- make breaking changes to the signature of a function: the types or order of its existing parameters, its return type,
+  its `noexcept` or `constexpr` specifier, or the const-ness of a member function. New parameters may be added if they
+  have a default value;
+- remove or rename a function or class, or change the template parameters of a public class template;
 - change which exceptions a function throws, or the [exception ids](../home/exceptions.md);
-- change access specifiers or default arguments.
+- change access specifiers, or change or remove existing default arguments. New default arguments may be added;
+- change the JSON type that a valid input parses to, or the text that `dump()` produces for a valid value;
+- accept input that was rejected before, or reject input that was accepted before;
+- change the order in which the keys of an object are iterated. The default type sorts keys, and
+  [`ordered_json`](../api/ordered_json.md) keeps insertion order;
+- change when iterators, pointers, or references are invalidated, or the state of a moved-from `basic_json`;
+- add or remove implicit conversions from `basic_json`;
+- change how `to_json` and `from_json` functions are found, or the behavior of
+  [`adl_serializer`](../api/adl_serializer/index.md);
+- add pure virtual functions to the [`json_sax`](../api/json_sax/index.md) interface;
+- remove, rename, renumber, or add enumerators of `value_t`;
+- remove or rename a documented macro, CMake option, CMake target, or header, or change what a documented macro does.
 
 Exceptions to these rules, for instance when fixing a bug requires changing the exception a function throws, are
 documented in the [release notes](../home/releases.md).
@@ -51,13 +64,14 @@ The following are **not** part of the public API and may change in any release,
 
 - The text of exception messages returned by `what()`. Use the [exception id](../home/exceptions.md) to tell errors
   apart.
-- The ABI, including `sizeof(basic_json)` and the memory layout of its values. Recompile your code when you upgrade the
-  library. The [versioned inline namespace](../features/namespace.md) turns mixing versions into a link error.
+- The ABI, including `sizeof(basic_json)` and the memory layout of its values. The
+  [versioned inline namespace](../features/namespace.md) turns mixing versions into a link error.
+- The hash values returned by `std::hash` for `basic_json`. Numbers that compare equal still hash equally.
 - Everything in namespace `nlohmann::detail`, and macros and type traits that are not documented in the
   [API reference](../api/basic_json/index.md).
 
-Changes that would break the public API are only added behind a macro whose default keeps the 3.x behavior, see
-[Version 4.0](#version-40).
+Breaking changes are only added behind a macro whose default keeps the 3.x behavior. See [Version 4.0](#version-40) and
+the [macro overview](../features/macros.md).
 
 ## Version 4.0
 

docs/mkdocs/docs/integration/package_managers.md

@@ -125,10 +125,24 @@ meson wrap install nlohmann_json
 Please see the Meson project for any issues regarding the packaging.
 
 The provided `meson.build` can also be used as an alternative to CMake for installing `nlohmann_json` system-wide in
-which case a [pkg-config](pkg-config.md) file is installed. To use it, have your build system require the
-`nlohmann_json` pkg-config dependency. In Meson, it is preferred to use the
-[`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a subproject fallback, rather than
-using the subproject directly.
+which case a [pkg-config](pkg-config.md) file and the CMake package config files are installed. To use it, have your build system require
+the `nlohmann_json` pkg-config dependency, or use [`find_package(nlohmann_json)`](cmake.md#external) in CMake. In Meson,
+it is preferred to use the [`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a
+subproject fallback, rather than using the subproject directly.
+
+The options that change the library's configuration are available in Meson as well, named like the
+[CMake options](cmake.md#cmake-options) without the `JSON_` prefix: `MultipleHeaders`, `GlobalUDLs`,
+`ImplicitConversions`, `DisableEnumSerialization`, `DisableTupleReferenceConversion`, `Diagnostics`,
+`Diagnostic_Positions`, `LegacyDiscardedValueComparison`, `StrictNulHandling`, `StrictBinaryUTF8`, and
+`DeleteDeprecatedFunctions`. They have the same defaults as in CMake, except that
+`MultipleHeaders` is `false`. Set them with `-D` when setting up the build, or with the subproject name as prefix when
+the library is used as a subproject:
+
+```shell
+meson setup build -Dnlohmann_json:Diagnostics=true
+```
+
+The resulting compile definitions are part of the Meson dependency, the pkg-config file, and the CMake target.
 
 ??? example "Example: Wrap"
 

include/nlohmann/detail/json_pointer.hpp

@@ -473,19 +473,15 @@ class json_pointer
             // convert null values to arrays or objects before continuing
             if (ptr->is_null())
             {
-                // check if the reference token is a valid array index, that is
-                // a nonempty sequence of digits without a leading '0'
-                // (cf. RFC 6901, Sect. 4); tokens that could never be a valid
-                // array index (such as "01" or "") are treated as object keys
-                const bool nums = !reference_token.empty()
-                                  && (reference_token.size() == 1 || reference_token[0] != '0')
-                                  && std::all_of(reference_token.begin(), reference_token.end(),
-                                                 [](const unsigned char x)
+                // check if the reference token is a number
+                const bool nums =
+                    std::all_of(reference_token.begin(), reference_token.end(),
+                                [](const unsigned char x)
                 {
                     return std::isdigit(x);
                 });
 
-                // change value to an array for array indices or "-" or to object otherwise
+                // change value to an array for numbers or "-" or to object otherwise
                 *ptr = (nums || reference_token == "-")
                        ? detail::value_t::array
                        : detail::value_t::object;

meson.build

@@ -2,24 +2,120 @@ project('nlohmann_json',
     'cpp',
     version : '3.12.0',
     license : 'MIT',
+    meson_version : '>= 0.64',
+    default_options: ['cpp_std=c++11'],
 )
 
+if get_option('MultipleHeaders')
+    incdir = 'include'
+else
+    incdir = 'single_include'
+endif
+
+# The same compile definitions as the CMake target (see target_compile_definitions
+# in CMakeLists.txt): only an option that differs from its default adds one.
+json_defines = []
+if not get_option('GlobalUDLs')
+    json_defines += 'JSON_USE_GLOBAL_UDLS=0'
+endif
+if not get_option('ImplicitConversions')
+    json_defines += 'JSON_USE_IMPLICIT_CONVERSIONS=0'
+endif
+if get_option('DisableEnumSerialization')
+    json_defines += 'JSON_DISABLE_ENUM_SERIALIZATION=1'
+endif
+if get_option('DisableTupleReferenceConversion')
+    json_defines += 'JSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1'
+endif
+if get_option('Diagnostics')
+    json_defines += 'JSON_DIAGNOSTICS=1'
+endif
+if get_option('Diagnostic_Positions')
+    json_defines += 'JSON_DIAGNOSTIC_POSITIONS=1'
+endif
+if get_option('LegacyDiscardedValueComparison')
+    json_defines += 'JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1'
+endif
+if get_option('StrictNulHandling')
+    json_defines += 'JSON_STRICT_NUL_HANDLING=1'
+endif
+if get_option('StrictBinaryUTF8')
+    json_defines += 'JSON_STRICT_BINARY_UTF8=1'
+endif
+if get_option('DeleteDeprecatedFunctions')
+    json_defines += 'JSON_DELETE_DEPRECATED_FUNCTIONS=1'
+endif
+
+cpp_args = []
+foreach define : json_defines
+    cpp_args += '-D' + define
+endforeach
+
 nlohmann_json_dep = declare_dependency(
-    include_directories: include_directories('single_include')
+    compile_args: cpp_args,
+    include_directories: include_directories(incdir)
 )
+meson.override_dependency('nlohmann_json', nlohmann_json_dep)
 
+# The multi-header version under the name earlier versions of this file used
 nlohmann_json_multiple_headers = declare_dependency(
+    compile_args: cpp_args,
     include_directories: include_directories('include')
 )
 
 if not meson.is_subproject()
-install_headers('single_include/nlohmann/json.hpp', subdir: 'nlohmann')
-install_headers('single_include/nlohmann/json_fwd.hpp', subdir: 'nlohmann')
-install_headers('single_include/nlohmann/json_literals.hpp', subdir: 'nlohmann')
+    install_subdir(
+        incdir / 'nlohmann',
+        install_dir: get_option('includedir'),
+        install_tag: 'devel',
+    )
 
-pkgc = import('pkgconfig')
-pkgc.generate(name: 'nlohmann_json',
-    version: meson.project_version(),
-    description: 'JSON for Modern C++'
-)
+    pkgc = import('pkgconfig')
+    pkgc.generate(name: 'nlohmann_json',
+        version: meson.project_version(),
+        description: 'JSON for Modern C++',
+        extra_cflags: cpp_args,
+        install_dir: get_option('datadir') / 'pkgconfig',
+    )
+
+    # CMake package config files, so that find_package(nlohmann_json) works. The
+    # include directory is given relative to the config files, so that the
+    # installation can be relocated. This is not possible if includedir or datadir
+    # is an absolute path outside the prefix (e.g., with the separate outputs of
+    # Nix); then the absolute include directory is used, as CMake does.
+    fs = import('fs')
+    cmake_install_dir = get_option('datadir') / 'cmake' / meson.project_name()
+    cmake_to_prefix = []
+    if fs.is_absolute(get_option('includedir')) or fs.is_absolute(cmake_install_dir)
+        cmake_include_dir = (get_option('prefix') / get_option('includedir')).replace('\\', '/')
+    else
+        foreach component : cmake_install_dir.split('/')
+            cmake_to_prefix += '..'
+        endforeach
+        cmake_include_dir = '${_IMPORT_PREFIX}/' + get_option('includedir')
+    endif
+
+    cmake_conf = configuration_data()
+    cmake_conf.set('PROJECT_NAME', meson.project_name())
+    cmake_conf.set('PROJECT_VERSION', meson.project_version())
+    cmake_conf.set('PROJECT_VERSION_MAJOR', meson.project_version().split('.')[0])
+    cmake_conf.set('NLOHMANN_JSON_TARGET_NAME', meson.project_name())
+    cmake_conf.set('NLOHMANN_JSON_TARGETS_EXPORT_NAME', meson.project_name() + 'Targets')
+    cmake_conf.set('NLOHMANN_JSON_INCLUDE_DIR', cmake_include_dir)
+    cmake_conf.set('NLOHMANN_JSON_CONFIG_TO_PREFIX', '/'.join(cmake_to_prefix))
+    cmake_conf.set('NLOHMANN_JSON_COMPILE_DEFINITIONS', ';'.join(json_defines))
+
+    foreach cmake_file : [
+        ['cmake/config.cmake.in', 'nlohmann_jsonConfig.cmake'],
+        ['cmake/nlohmann_jsonConfigVersion.cmake.in', 'nlohmann_jsonConfigVersion.cmake'],
+        ['cmake/nlohmann_jsonTargets.cmake.in', 'nlohmann_jsonTargets.cmake'],
+    ]
+        configure_file(
+            input: cmake_file[0],
+            output: cmake_file[1],
+            configuration: cmake_conf,
+            format: 'cmake@',
+            install_dir: cmake_install_dir,
+        )
+    endforeach
 endif

meson_options.txt

@@ -0,0 +1,66 @@
+option(
+    'MultipleHeaders',
+    type: 'boolean',
+    value: false,
+    description: 'Use non-amalgamated version of the library',
+)
+option(
+    'GlobalUDLs',
+    type: 'boolean',
+    value: true,
+    description: 'Place user-defined string literals in the global namespace',
+)
+option(
+    'ImplicitConversions',
+    type: 'boolean',
+    value: true,
+    description: 'Enable implicit conversions',
+)
+option(
+    'DisableEnumSerialization',
+    type: 'boolean',
+    value: false,
+    description: 'Disable default integer enum serialization',
+)
+option(
+    'DisableTupleReferenceConversion',
+    type: 'boolean',
+    value: false,
+    description: 'Disable conversion from a one-element tuple of a JSON reference',
+)
+option(
+    'Diagnostics',
+    type: 'boolean',
+    value: false,
+    description: 'Use extended diagnostic messages',
+)
+option(
+    'Diagnostic_Positions',
+    type: 'boolean',
+    value: false,
+    description: 'Enable diagnostic positions',
+)
+option(
+    'LegacyDiscardedValueComparison',
+    type: 'boolean',
+    value: false,
+    description: 'Enable legacy discarded value comparison',
+)
+option(
+    'StrictNulHandling',
+    type: 'boolean',
+    value: false,
+    description: 'Enable strict NUL-byte handling',
+)
+option(
+    'StrictBinaryUTF8',
+    type: 'boolean',
+    value: false,
+    description: 'Enable UTF-8 checks in the CBOR, UBJSON, BJData, and BSON writers',
+)
+option(
+    'DeleteDeprecatedFunctions',
+    type: 'boolean',
+    value: false,
+    description: 'Delete the deprecated functions instead of only deprecating them',
+)

single_include/nlohmann/json.hpp

@@ -20382,19 +20382,15 @@ class json_pointer
             // convert null values to arrays or objects before continuing
             if (ptr->is_null())
             {
-                // check if the reference token is a valid array index, that is
-                // a nonempty sequence of digits without a leading '0'
-                // (cf. RFC 6901, Sect. 4); tokens that could never be a valid
-                // array index (such as "01" or "") are treated as object keys
-                const bool nums = !reference_token.empty()
-                                  && (reference_token.size() == 1 || reference_token[0] != '0')
-                                  && std::all_of(reference_token.begin(), reference_token.end(),
-                                                 [](const unsigned char x)
+                // check if the reference token is a number
+                const bool nums =
+                    std::all_of(reference_token.begin(), reference_token.end(),
+                                [](const unsigned char x)
                 {
                     return std::isdigit(x);
                 });
 
-                // change value to an array for array indices or "-" or to object otherwise
+                // change value to an array for numbers or "-" or to object otherwise
                 *ptr = (nums || reference_token == "-")
                        ? detail::value_t::array
                        : detail::value_t::object;

tests/src/unit-json_pointer.cpp

@@ -466,72 +466,6 @@ TEST_CASE("JSON pointers")
         }
     }
 
-    SECTION("creating intermediate levels")
-    {
-        SECTION("tokens that are valid array indices create arrays")
-        {
-            json j;
-            j["/0"_json_pointer] = 1;
-            CHECK(j == json({1}));
-
-            json j2;
-            j2["/2"_json_pointer] = 1;
-            CHECK(j2 == json({nullptr, nullptr, 1}));
-
-            json j3;
-            j3["/-"_json_pointer] = 1;
-            CHECK(j3 == json({1}));
-
-            json j4;
-            j4["/foo/0/0"_json_pointer] = 1;
-            CHECK(j4 == json({{"foo", {{1}}}}));
-        }
-
-        SECTION("tokens that are no valid array indices create objects")
-        {
-            json j;
-            j["/one"_json_pointer] = 1;
-            CHECK(j == json({{"one", 1}}));
-
-            // leading '0' can never be a valid array index (RFC 6901, Sect. 4)
-            json j2;
-            j2["/01"_json_pointer] = 1;
-            CHECK(j2 == json({{"01", 1}}));
-
-            // the empty token is a valid object key, but no valid array index
-            json j3;
-            j3["/"_json_pointer] = 1;
-            CHECK(j3 == json({{"", 1}}));
-        }
-
-        SECTION("creating a level yields the same result as reusing it (#5357)")
-        {
-            json j;
-            j["/a/b/01/d"_json_pointer] = "value";
-
-            json j_init = json::object();
-            j_init["/a/b"_json_pointer] = json::object();
-            j_init["/a/b/01/d"_json_pointer] = "value";
-
-            const json expected = json::parse(R"({"a":{"b":{"01":{"d":"value"}}}})");
-            CHECK(j == expected);
-            CHECK(j_init == expected);
-
-            // unflatten uses the same key
-            const json flat = {{"/a/b/01/d", "value"}};
-            CHECK(flat.unflatten() == expected);
-        }
-
-        SECTION("existing arrays still reject invalid indices")
-        {
-            json j = {1, 2, 3};
-            CHECK_THROWS_WITH_AS(j["/01"_json_pointer],
-                                 "[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'", json::parse_error&);
-            CHECK_THROWS_WITH_AS(j.at("/01"_json_pointer),
-                                 "[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'", json::parse_error&);
-        }
-    }
-
     SECTION("flatten")
     {
         json j =

tools/check_build_options/README.md

@@ -0,0 +1,21 @@
+# check_build_options
+
+Checks that the Meson build and the pkg-config files offer the same options as the CMake target, so that a new CMake
+option is not forgotten in one of them.
+
+The compile definitions of the CMake target (`target_compile_definitions` in [`CMakeLists.txt`](../../CMakeLists.txt))
+are the reference. When you add an option there, also add it to
+
+- the pkg-config block in `CMakeLists.txt` (`NLOHMANN_JSON_PKGCONFIG_CFLAGS`),
+- [`meson_options.txt`](../../meson_options.txt), named without the `JSON_` prefix and with the same default,
+- [`meson.build`](../../meson.build) (`json_defines`), and
+- the list of Meson options in
+  [`docs/mkdocs/docs/integration/package_managers.md`](../../docs/mkdocs/docs/integration/package_managers.md).
+
+Run the check with
+
+```shell
+make check_build_options
+```
+
+It needs only Python 3 and runs in the `ci_meson_install` CI job.

tools/check_build_options/check_build_options.py

@@ -0,0 +1,144 @@
+#!/usr/bin/env python3
+"""Check that the Meson build and the pkg-config files offer the CMake options.
+
+The compile definitions of the CMake target (target_compile_definitions in
+CMakeLists.txt) are the reference. For every option used there, the script
+checks that
+
+- the CMake pkg-config file adds the same definition under the same condition,
+- meson_options.txt has a boolean option of the same name without the "JSON_"
+  prefix and with the same default,
+- meson.build adds the same definition under the same condition, and
+- the Meson section of the package manager documentation lists the option.
+
+Meson's MultipleHeaders option selects the include directory and adds no
+definition; it is the only Meson option without a definition.
+"""
+
+import argparse
+import os
+import re
+import sys
+
+REPO_ROOT = os.path.normpath(os.path.join(sys.path[0], '..', '..'))
+DOCS = os.path.join('docs', 'mkdocs', 'docs', 'integration', 'package_managers.md')
+
+# Meson options that add no compile definition, with their default
+MESON_ONLY = {'MultipleHeaders': 'false'}
+
+
+def read(root, path):
+    with open(os.path.join(root, path), encoding='utf-8') as f:
+        return f.read()
+
+
+def cmake_target_definitions(cmake):
+    """Return {option: (definition, add_if_on)} from target_compile_definitions."""
+    block = re.search(r'target_compile_definitions\(\s*\$\{NLOHMANN_JSON_TARGET_NAME\}\s*INTERFACE(.*?)\n\)', cmake, re.S)
+    if not block:
+        sys.exit('CMakeLists.txt: target_compile_definitions of the target not found')
+    result = {}
+    for line in block.group(1).split('\n'):
+        line = line.strip()
+        if not line:
+            continue
+        m = re.fullmatch(r'\$<\$<NOT:\$<BOOL:\$\{JSON_(\w+)\}>>:(\w+=\w+)>', line)
+        if m:
+            result[m.group(1)] = (m.group(2), False)
+            continue
+        m = re.fullmatch(r'\$<\$<BOOL:\$\{JSON_(\w+)\}>:(\w+=\w+)>', line)
+        if m:
+            result[m.group(1)] = (m.group(2), True)
+            continue
+        sys.exit(f'CMakeLists.txt: unexpected line in target_compile_definitions: {line}')
+    return result
+
+
+def cmake_defaults(cmake):
+    """Return {option: 'true'/'false'} for the option() calls of the form JSON_<name>."""
+    return {m.group(1): 'true' if m.group(2) == 'ON' else 'false'
+            for m in re.finditer(r'^option\(JSON_(\w+)\s+"[^"]*"\s+(ON|OFF)\)', cmake, re.M)}
+
+
+def cmake_pkgconfig_definitions(cmake):
+    """Return {option: (definition, add_if_on)} from the pkg-config block."""
+    return {m.group(2): (m.group(3), m.group(1) is None)
+            for m in re.finditer(r'if \((NOT )?JSON_(\w+)\)\s*\n\s*string\(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -D(\w+=\w+)"\)', cmake)}
+
+
+def meson_options(options):
+    """Return {option: default} for the boolean options in meson_options.txt."""
+    result = {}
+    for block in re.findall(r'option\((.*?)\)', options, re.S):
+        name = re.search(r"'(\w+)'", block).group(1)
+        kind = re.search(r"type\s*:\s*'(\w+)'", block)
+        value = re.search(r'value\s*:\s*(\w+)', block)
+        result[name] = value.group(1) if kind and kind.group(1) == 'boolean' and value else None
+    return result
+
+
+def meson_definitions(meson):
+    """Return {option: (definition, add_if_on)} from meson.build."""
+    return {m.group(2): (m.group(3), m.group(1) is None)
+            for m in re.finditer(r"if (not )?get_option\('(\w+)'\)\s*\n\s*json_defines \+= '(\w+=\w+)'", meson)}
+
+
+def describe(definition):
+    name, add_if_on = definition
+    return f'{name} if {"enabled" if add_if_on else "disabled"}'
+
+
+def compare(errors, where, expected, actual):
+    for option, definition in expected.items():
+        if option not in actual:
+            errors.append(f'{where}: no definition for option {option} (expected {describe(definition)})')
+        elif actual[option] != definition:
+            errors.append(f'{where}: option {option} adds {describe(actual[option])}, expected {describe(definition)}')
+    for option in actual.keys() - expected.keys():
+        errors.append(f'{where}: definition for option {option}, which the CMake target does not have')
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__.split('\n')[0])
+    parser.add_argument('root', nargs='?', default=REPO_ROOT, help='repository root (default: %(default)s)')
+    root = parser.parse_args().root
+
+    cmake = read(root, 'CMakeLists.txt')
+    reference = cmake_target_definitions(cmake)
+    defaults = cmake_defaults(cmake)
+    errors = []
+
+    compare(errors, 'CMakeLists.txt (pkg-config)', reference, cmake_pkgconfig_definitions(cmake))
+    compare(errors, 'meson.build', reference, meson_definitions(read(root, 'meson.build')))
+
+    options = meson_options(read(root, 'meson_options.txt'))
+    for option in reference:
+        if option not in defaults:
+            errors.append(f'CMakeLists.txt: no option(JSON_{option} ... ON|OFF)')
+        elif option not in options:
+            errors.append(f'meson_options.txt: option {option} missing')
+        elif options[option] != defaults[option]:
+            errors.append(f'meson_options.txt: option {option} must be boolean with value {defaults[option]} as in CMake')
+    for option, default in MESON_ONLY.items():
+        if options.get(option) != default:
+            errors.append(f'meson_options.txt: option {option} must be boolean with value {default}')
+    for option in options.keys() - reference.keys() - MESON_ONLY.keys():
+        errors.append(f'meson_options.txt: option {option} has no counterpart in the CMake target')
+
+    docs = read(root, DOCS)
+    for option in sorted(set(options) & (reference.keys() | MESON_ONLY.keys())):
+        if f'`{option}`' not in docs:
+            errors.append(f'{DOCS}: Meson option {option} not listed')
+
+    for error in errors:
+        print(error, file=sys.stderr)
+    if errors:
+        print('The Meson build and the pkg-config files must offer the options of the CMake target; see '
+              'tools/check_build_options/README.md.', file=sys.stderr)
+        return 1
+    print(f'OK: {len(reference)} options with compile definitions agree between CMake, pkg-config, and Meson.')
+    return 0
+
+
+if __name__ == '__main__':
+    sys.exit(main())