Add missing diagnostic-positions test coverage (lifetime, input adapters, SAX)

Building on the merged unit-class_parser.cpp from #5417, add
characterization tests (regression protection for existing behavior, not a
behavior change) for JSON_DIAGNOSTIC_POSITIONS:

- value lifetime: copy ctor copies positions recursively, move ctor resets
  the moved-from value to npos, and mutating a parsed document (operator[],
  push_back, erase) leaves the parent's stale span and siblings' positions
  untouched while new values get npos.
- input adapters: wide-string input positions count transcoded UTF-8 bytes
  (not wide characters), BOM-prefixed input's start_pos() reflects the
  skipped 3-byte BOM, istringstream/ifstream/iterator-pair inputs report
  consistent (non-npos) positions, and binary formats (CBOR, MessagePack,
  UBJSON, BSON) always report npos.
- a user-constructed json_sax_dom_parser with no lexer (as used when driving
  json::sax_parse() directly) reports npos for every value, since it has no
  m_lexer_ref to source positions from.

While characterizing swap(), found that basic_json::swap() (and the friend
swap() that forwards to it) does not swap start_position/end_position,
unlike copy-assignment's operator=(basic_json), which does as part of its
copy-and-swap implementation. This looks like a real inconsistency/bug, but
per the scope of this test-only change it is only pinned (not fixed) here;
see the comment at the "swap() does NOT exchange positions" section.

Fixes #5420

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

Makefile

@@ -1,4 +1,4 @@
-.PHONY: pretty clean ChangeLog.md release update_hedley update_hedley_undef
+.PHONY: pretty clean ChangeLog.md release
 
 ##########################################################################
 # configuration
@@ -41,8 +41,6 @@ all:
 	@echo "fuzz_testing_ubjson - prepare fuzz testing of the UBJSON parser"
 	@echo "pretty - beautify code with Artistic Style"
 	@echo "run_benchmarks - build and run benchmarks"
-	@echo "update_hedley - download Hedley and regenerate hedley.hpp / hedley_undef.hpp"
-	@echo "update_hedley_undef - rebuild hedley_undef.hpp from the JSON_HEDLEY_* #define names in hedley.hpp"
 
 
 ##########################################################################
@@ -243,24 +241,11 @@ update_hedley:
 	rm -f include/nlohmann/thirdparty/hedley/hedley.hpp include/nlohmann/thirdparty/hedley/hedley_undef.hpp
 	curl https://raw.githubusercontent.com/nemequ/hedley/master/hedley.h -o include/nlohmann/thirdparty/hedley/hedley.hpp
 	$(SED) -i 's/HEDLEY_/JSON_HEDLEY_/g' include/nlohmann/thirdparty/hedley/hedley.hpp
+	grep "[[:blank:]]*#[[:blank:]]*undef" include/nlohmann/thirdparty/hedley/hedley.hpp | grep -v "__" | sort | uniq | $(SED) 's/ //g' | $(SED) 's/undef/undef /g' > include/nlohmann/thirdparty/hedley/hedley_undef.hpp
 	$(SED) -i '1s/^/#pragma once\n\n/' include/nlohmann/thirdparty/hedley/hedley.hpp
-	$(MAKE) update_hedley_undef
+	$(SED) -i '1s/^/#pragma once\n\n/' include/nlohmann/thirdparty/hedley/hedley_undef.hpp
 	$(MAKE) amalgamate
 
-# Rebuild hedley_undef.hpp from every JSON_HEDLEY_* name that hedley.hpp
-# #defines. Hedley does not #undef all of its public macros internally (see
-# #5408), so grepping those #undef lines misses names such as
-# JSON_HEDLEY_PRAGMA. cmake/scripts/gen_hedley_undef_check.cmake is the
-# single source of truth for this extraction (tests/CMakeLists.txt uses the
-# same script, in MODE=checks, to generate the matching leak-check test), so
-# the vendored header, the generated #undef list, and the regression test
-# cannot drift apart.
-update_hedley_undef:
-	cmake -DHEDLEY_HPP=include/nlohmann/thirdparty/hedley/hedley.hpp \
-	      -DOUTPUT=include/nlohmann/thirdparty/hedley/hedley_undef.hpp \
-	      -DMODE=undef \
-	      -P cmake/scripts/gen_hedley_undef_check.cmake
-
 ##########################################################################
 # serve_header.py
 ##########################################################################

cmake/scripts/gen_hedley_undef_check.cmake

@@ -1,105 +0,0 @@
-# Shared extractor for the JSON_HEDLEY_* macro names defined in hedley.hpp.
-#
-# Every macro that hedley.hpp #defines must be #undef-ed again once json.hpp
-# has been fully processed (see include/nlohmann/detail/macro_unscope.hpp
-# and https://github.com/nlohmann/json/issues/5408). Deriving the macro list
-# straight from hedley.hpp here -- instead of hand-maintaining it in two
-# places -- means hedley_undef.hpp and the regression test that checks for
-# leaked macros can never drift apart, even after a future `make
-# update_hedley` pulls in new macros from upstream Hedley.
-#
-#   MODE=undef  (default): write hedley_undef.hpp (SPDX header, #pragma once,
-#               one #undef per macro name) -- used by `make update_hedley_undef`
-#   MODE=checks:            write one #ifdef/FAIL_CHECK/#endif per macro name,
-#               meant to be #include-d inside a TEST_CASE -- used by
-#               tests/CMakeLists.txt to (re)generate the include for
-#               tests/src/unit-no-macro-leak.cpp
-#
-# Required variables:
-#   HEDLEY_HPP  path to include/nlohmann/thirdparty/hedley/hedley.hpp
-#   OUTPUT      path of the file to (over)write
-# Optional:
-#   MODE        "undef" (default) or "checks"
-
-if(NOT DEFINED HEDLEY_HPP OR NOT DEFINED OUTPUT)
-    message(FATAL_ERROR "HEDLEY_HPP and OUTPUT must be set")
-endif()
-
-if(NOT EXISTS "${HEDLEY_HPP}")
-    message(FATAL_ERROR "Hedley header not found: ${HEDLEY_HPP}")
-endif()
-
-if(NOT DEFINED MODE)
-    set(MODE undef)
-endif()
-
-if(NOT MODE STREQUAL "undef" AND NOT MODE STREQUAL "checks")
-    message(FATAL_ERROR "MODE must be undef or checks, got: ${MODE}")
-endif()
-
-# Line-anchored, like `grep -oE "^[[:blank:]]*#[[:blank:]]*define[[:blank:]]+JSON_HEDLEY_[A-Za-z0-9_]+"`.
-# Unanchored matching would also pick up JSON_HEDLEY_* mentions inside
-# comments or string literals elsewhere in the file, which must not turn
-# into #undef lines.
-file(STRINGS "${HEDLEY_HPP}" hedley_lines)
-set(macro_names)
-foreach(line IN LISTS hedley_lines)
-    if("${line}" MATCHES "^[ \t]*#[ \t]*define[ \t]+(JSON_HEDLEY_[A-Za-z0-9_]+)")
-        list(APPEND macro_names "${CMAKE_MATCH_1}")
-    endif()
-endforeach()
-
-if(NOT macro_names)
-    message(FATAL_ERROR "No JSON_HEDLEY_* macros found in ${HEDLEY_HPP}")
-endif()
-
-list(REMOVE_DUPLICATES macro_names)
-# Lexicographic, locale-independent (ASCII-only names) -- matches `LC_ALL=C sort`.
-list(SORT macro_names COMPARE STRING)
-list(LENGTH macro_names macro_count)
-
-set(generated "")
-if(MODE STREQUAL "undef")
-    # Same banner `make update_hedley_undef` would stamp by hand, so the
-    # recipe is self-contained and its output is byte-stable across reruns.
-    string(APPEND generated "//     __ _____ _____ _____\n")
-    string(APPEND generated "//  __|  |   __|     |   | |  JSON for Modern C++\n")
-    string(APPEND generated "// |  |  |__   |  |  | | | |  version 3.12.0\n")
-    string(APPEND generated "// |_____|_____|_____|_|___|  https://github.com/nlohmann/json\n")
-    string(APPEND generated "//\n")
-    string(APPEND generated "// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>\n")
-    string(APPEND generated "// SPDX-License-Identifier: MIT\n")
-    string(APPEND generated "\n")
-    string(APPEND generated "#pragma once\n")
-    string(APPEND generated "\n")
-    foreach(name IN LISTS macro_names)
-        string(APPEND generated "#undef ${name}\n")
-    endforeach()
-else()
-    string(APPEND generated "// This file is generated by cmake/scripts/gen_hedley_undef_check.cmake\n")
-    string(APPEND generated "// from include/nlohmann/thirdparty/hedley/hedley.hpp. Do not edit it by\n")
-    string(APPEND generated "// hand -- it is regenerated on every build. ${macro_count} macros checked.\n\n")
-    foreach(name IN LISTS macro_names)
-        string(APPEND generated "#ifdef ${name}\n")
-        string(APPEND generated "    FAIL_CHECK(\"${name} leaked after including nlohmann/json.hpp\");\n")
-        string(APPEND generated "#endif\n")
-    endforeach()
-endif()
-
-get_filename_component(output_dir "${OUTPUT}" DIRECTORY)
-if(output_dir)
-    file(MAKE_DIRECTORY "${output_dir}")
-endif()
-
-# Avoid rewriting the file (and busting downstream incremental rebuilds)
-# when the content has not actually changed.
-set(write_output TRUE)
-if(EXISTS "${OUTPUT}")
-    file(READ "${OUTPUT}" existing_content)
-    if(existing_content STREQUAL generated)
-        set(write_output FALSE)
-    endif()
-endif()
-if(write_output)
-    file(WRITE "${OUTPUT}" "${generated}")
-endif()

include/nlohmann/thirdparty/hedley/hedley_undef.hpp

@@ -17,7 +17,7 @@
 #undef JSON_HEDLEY_CLANG_HAS_ATTRIBUTE
 #undef JSON_HEDLEY_CLANG_HAS_BUILTIN
 #undef JSON_HEDLEY_CLANG_HAS_CPP_ATTRIBUTE
-#undef JSON_HEDLEY_CLANG_HAS_DECLSPEC_ATTRIBUTE
+#undef JSON_HEDLEY_CLANG_HAS_DECLSPEC_DECLSPEC_ATTRIBUTE
 #undef JSON_HEDLEY_CLANG_HAS_EXTENSION
 #undef JSON_HEDLEY_CLANG_HAS_FEATURE
 #undef JSON_HEDLEY_CLANG_HAS_WARNING
@@ -108,10 +108,7 @@
 #undef JSON_HEDLEY_PELLES_VERSION_CHECK
 #undef JSON_HEDLEY_PGI_VERSION
 #undef JSON_HEDLEY_PGI_VERSION_CHECK
-#undef JSON_HEDLEY_PRAGMA
 #undef JSON_HEDLEY_PREDICT
-#undef JSON_HEDLEY_PREDICT_FALSE
-#undef JSON_HEDLEY_PREDICT_TRUE
 #undef JSON_HEDLEY_PRINTF_FORMAT
 #undef JSON_HEDLEY_PRIVATE
 #undef JSON_HEDLEY_PUBLIC

single_include/nlohmann/json.hpp

@@ -26965,7 +26965,7 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
 #undef JSON_HEDLEY_CLANG_HAS_ATTRIBUTE
 #undef JSON_HEDLEY_CLANG_HAS_BUILTIN
 #undef JSON_HEDLEY_CLANG_HAS_CPP_ATTRIBUTE
-#undef JSON_HEDLEY_CLANG_HAS_DECLSPEC_ATTRIBUTE
+#undef JSON_HEDLEY_CLANG_HAS_DECLSPEC_DECLSPEC_ATTRIBUTE
 #undef JSON_HEDLEY_CLANG_HAS_EXTENSION
 #undef JSON_HEDLEY_CLANG_HAS_FEATURE
 #undef JSON_HEDLEY_CLANG_HAS_WARNING
@@ -27056,10 +27056,7 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
 #undef JSON_HEDLEY_PELLES_VERSION_CHECK
 #undef JSON_HEDLEY_PGI_VERSION
 #undef JSON_HEDLEY_PGI_VERSION_CHECK
-#undef JSON_HEDLEY_PRAGMA
 #undef JSON_HEDLEY_PREDICT
-#undef JSON_HEDLEY_PREDICT_FALSE
-#undef JSON_HEDLEY_PREDICT_TRUE
 #undef JSON_HEDLEY_PRINTF_FORMAT
 #undef JSON_HEDLEY_PRIVATE
 #undef JSON_HEDLEY_PUBLIC

tests/CMakeLists.txt

@@ -125,51 +125,6 @@ json_test_set_test_options(test-unicode4 TEST_PROPERTIES TIMEOUT 3000)
 # add unit tests
 #############################################################################
 
-# Generate the leak checks for every JSON_HEDLEY_* macro defined in
-# hedley.hpp; tests/src/unit-no-macro-leak.cpp #include-s the result after
-# nlohmann/json.hpp (see issue #5408). Using the shared
-# cmake/scripts/gen_hedley_undef_check.cmake script (also used by `make
-# update_hedley_undef`) instead of a hand-maintained list of macro names
-# means this test can never go stale after a future `make update_hedley`.
-set(hedley_hpp "${PROJECT_SOURCE_DIR}/include/nlohmann/thirdparty/hedley/hedley.hpp")
-set(hedley_undef_check_script "${PROJECT_SOURCE_DIR}/cmake/scripts/gen_hedley_undef_check.cmake")
-set(hedley_undef_checks "${PROJECT_BINARY_DIR}/include/hedley_undef_checks.inc")
-
-# Reconfigure whenever the vendored header or the generator script changes,
-# so a `cmake --build` after `make update_hedley` does not silently keep a
-# stale generated file around.
-set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS
-    "${hedley_hpp}"
-    "${hedley_undef_check_script}")
-
-# Generate once at configure time, so the very first build (before any
-# custom-command build step has run) already has an up-to-date file.
-execute_process(
-    COMMAND ${CMAKE_COMMAND}
-        "-DHEDLEY_HPP=${hedley_hpp}"
-        "-DOUTPUT=${hedley_undef_checks}"
-        -DMODE=checks
-        -P "${hedley_undef_check_script}"
-    RESULT_VARIABLE hedley_undef_check_result
-)
-if(NOT hedley_undef_check_result EQUAL 0)
-    message(FATAL_ERROR "Failed to generate ${hedley_undef_checks}")
-endif()
-
-# Also (re)generate as a build step, so an incremental build after editing
-# hedley.hpp without a full reconfigure still picks up the change.
-add_custom_command(
-    OUTPUT "${hedley_undef_checks}"
-    COMMAND ${CMAKE_COMMAND}
-        "-DHEDLEY_HPP=${hedley_hpp}"
-        "-DOUTPUT=${hedley_undef_checks}"
-        -DMODE=checks
-        -P "${hedley_undef_check_script}"
-    DEPENDS "${hedley_hpp}" "${hedley_undef_check_script}"
-    COMMENT "Generating Hedley undef leak checks"
-    VERBATIM)
-add_custom_target(generate_hedley_undef_checks DEPENDS "${hedley_undef_checks}")
-
 if("${JSON_TestStandards}" STREQUAL "")
     set(test_cxx_standards 11 14 17 20 23)
     unset(test_force)
@@ -208,14 +163,6 @@ foreach(file ${files})
     json_test_add_test_for(${file} MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force})
 endforeach()
 
-# 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.
-foreach(cxx_standard ${test_cxx_standards})
-    if(TARGET test-no-macro-leak_cpp${cxx_standard})
-        add_dependencies(test-no-macro-leak_cpp${cxx_standard} generate_hedley_undef_checks)
-    endif()
-endforeach()
-
 if(json_32bit_test_only)
     # Skip all other tests in this file
     return()
@@ -230,6 +177,24 @@ json_test_add_test_for(src/unit-comparison.cpp
     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
+    COMPILE_DEFINITIONS JSON_DIAGNOSTIC_POSITIONS=1
+)
+json_test_add_test_for(src/unit-class_parser.cpp
+    NAME test-class_parser_diagnostic_positions
+    MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
+)
+
+# test diagnostic positions again without regular diagnostics (JSON pointer paths)
+json_test_set_test_options(test-diagnostic-positions_only
+    COMPILE_DEFINITIONS JSON_DIAGNOSTICS=0
+)
+json_test_add_test_for(src/unit-diagnostic-positions.cpp
+    NAME test-diagnostic-positions_only
+    MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
+)
+
 # *DO NOT* use json_test_set_test_options() below this line
 
 #############################################################################

tests/src/unit-class_parser.cpp

@@ -17,6 +17,8 @@ using nlohmann::json;
 
 #include <valarray>
 #include <algorithm>
+#include <cstdio>
+#include <fstream>
 #include <list>
 #include <sstream>
 #include <string>
@@ -344,6 +346,50 @@ void trailing_comma_helper(const std::string& s)
     }
 }
 
+#if JSON_DIAGNOSTIC_POSITIONS
+/**
+ * Validates that the generated JSON object is the same as expected
+ * Validates that the start position and end position match the start and end of the string
+ *
+ * This check assumes that there is no whitespace around the json object in the original string.
+ */
+void validate_generated_json_and_start_end_pos_helper(const std::string& original_string, const json& j, const json& check)
+{
+    CHECK(j == check);
+    CHECK(j.start_pos() == 0);
+    CHECK(j.end_pos() == original_string.size());
+}
+
+/**
+ * Parses the root object from the given root string and validates that the start and end positions for the nested object are correct.
+ *
+ * This checks that whitespace around the nested object is included in the start and end positions of the root object.
+ */
+void validate_start_end_pos_for_nested_obj_helper(const std::string& nested_type_json_str, const std::string& root_type_json_str, const json& expected_json, const json::parser_callback_t& cb = nullptr)
+{
+    json j;
+
+    // 1. If callback is provided, use callback version of parse()
+    if (cb)
+    {
+        j = json::parse(root_type_json_str, cb);
+    }
+    else
+    {
+        j = json::parse(root_type_json_str);
+    }
+
+    // 2. Check if the generated JSON is as expected
+    // Assumptions: The root_type_json_str does not have any whitespace around the json object
+    validate_generated_json_and_start_end_pos_helper(root_type_json_str, j, expected_json);
+
+    // 3. Get the nested object
+    const auto& nested = j["nested"];
+    // 4. Check if the start and end positions are generated correctly for nested objects and arrays
+    CHECK(nested_type_json_str == root_type_json_str.substr(nested.start_pos(), nested.end_pos() - nested.start_pos()));
+}
+#endif
+
 } // namespace
 
 TEST_CASE("parser class")
@@ -1779,6 +1825,228 @@ TEST_CASE("parser class")
         CHECK_THROWS_WITH_AS(_ = json::parse("/a", nullptr, true, true), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid comment; expecting '/' or '*' after '/'; last read: '/a'", json::parse_error);
         CHECK_THROWS_WITH_AS(_ = json::parse("/*", nullptr, true, true), "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid comment; missing closing '*/'; last read: '/*<U+0000>'", json::parse_error);
     }
+
+#if JSON_DIAGNOSTIC_POSITIONS
+    // Macro for all test cases for start_pos and end_pos
+#define SETUP_TESTCASES() \
+    SECTION("with callback") \
+    { \
+        SECTION("filter nothing") \
+        { \
+            json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t /*unused*/, json& /*unused*/) noexcept \
+            { \
+                return true; \
+            }; \
+            validate_start_end_pos_for_nested_obj_helper(nested_type_json_str, root_type_json_str, expected, cb); \
+        } \
+        SECTION("filter element") \
+        { \
+            json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t event, json& j) noexcept \
+            { \
+                return (event != json::parse_event_t::key && event != json::parse_event_t::value) || j != json("a"); \
+            }; \
+            validate_start_end_pos_for_nested_obj_helper(nested_type_json_str, root_type_json_str, filteredExpected, cb); \
+        } \
+    } \
+    SECTION("without callback") \
+    { \
+        validate_start_end_pos_for_nested_obj_helper(nested_type_json_str, root_type_json_str, expected); \
+    }
+
+    SECTION("retrieve start position and end position")
+    {
+        SECTION("for object")
+        {
+            // Create an object with spaces to test the start and end positions. Spaces will not be included in the
+            // JSON object, however, the start and end positions should include the spaces from the input JSON string.
+            const std::string nested_type_json_str =  R"({    "a":       1,"b"      : "test1"})";
+            const std::string root_type_json_str =  R"({    "nested": )" + nested_type_json_str + R"(, "anotherValue": "test2"})";
+            auto expected = json({{"nested", {{"a", 1}, {"b", "test1"}}}, {"anotherValue", "test2"}});
+            auto filteredExpected = expected;
+            filteredExpected["nested"].erase("a");
+
+            SETUP_TESTCASES()
+        }
+
+        SECTION("for array")
+        {
+            const std::string nested_type_json_str =  R"(["a", "test", 45])";
+            const std::string root_type_json_str =  R"({   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
+            auto expected = json({{"nested", {"a", "test", 45}}, {"anotherValue", "test"}});
+            auto filteredExpected = expected;
+            filteredExpected["nested"] = json({"test", 45});
+            SETUP_TESTCASES()
+        }
+
+        SECTION("for array with objects")
+        {
+            const std::string nested_type_json_str =  R"([{"a": 1, "b": "test"}, {"c": 2, "d": "test2"}])";
+            const std::string root_type_json_str =  R"({   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
+            auto expected = json({{"nested", {{{"a", 1}, {"b", "test"}}, {{"c", 2}, {"d", "test2"}}}}, {"anotherValue", "test"}});
+            auto filteredExpected = expected;
+            filteredExpected["nested"][0].erase("a");
+            SETUP_TESTCASES()
+
+            auto j = json::parse(root_type_json_str);
+            auto nested_array = j["nested"];
+            const auto& nested_obj = nested_array[0];
+            CHECK(nested_type_json_str.substr(1, 21) == root_type_json_str.substr(nested_obj.start_pos(), nested_obj.end_pos() - nested_obj.start_pos()));
+            CHECK(nested_type_json_str.substr(24, 22) == root_type_json_str.substr(nested_array[1].start_pos(), nested_array[1].end_pos() - nested_array[1].start_pos()));
+        }
+
+        SECTION("for two levels of nesting objects")
+        {
+            const std::string nested_type_json_str =  R"({"nested2": {"b": "test"}})";
+            const std::string root_type_json_str =  R"({   "a": 2, "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
+            auto expected = json({{"a", 2}, {"nested", {{"nested2", {{"b", "test"}}}}}, {"anotherValue", "test"}});
+            auto filteredExpected = expected;
+            filteredExpected.erase("a");
+            SETUP_TESTCASES()
+
+            auto j = json::parse(root_type_json_str);
+            auto nested_obj = j["nested"]["nested2"];
+            CHECK(nested_type_json_str.substr(12, 13) == root_type_json_str.substr(nested_obj.start_pos(), nested_obj.end_pos() - nested_obj.start_pos()));
+        }
+
+        SECTION("for simple types")
+        {
+            SECTION("no nested")
+            {
+                SECTION("with callback")
+                {
+                    json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t /*unused*/, json& /*unused*/) noexcept
+                    {
+                        return true;
+                    };
+
+                    // 1. string type
+                    std::string json_str =  R"("test")";
+                    auto j = json::parse(json_str, cb);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, "test");
+
+                    // 2. number type
+                    json_str =  R"(1)";
+                    j = json::parse(json_str, cb);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, 1);
+
+                    // 3. boolean type
+                    json_str =  R"(true)";
+                    j = json::parse(json_str, cb);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, true);
+
+                    // 4. null type
+                    json_str =  R"(null)";
+                    j = json::parse(json_str, cb);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, nullptr);
+                }
+
+                SECTION("without callback")
+                {
+                    // 1. string type
+                    std::string json_str =  R"("test")";
+                    auto j = json::parse(json_str);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, "test");
+
+                    // 2. number type
+                    json_str =  R"(1)";
+                    j = json::parse(json_str);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, 1);
+
+                    json_str = R"(1.001239923)";
+                    j = json::parse(json_str);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, 1.001239923);
+
+                    json_str = R"(1.123812389000000)";
+                    j = json::parse(json_str);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, 1.123812389);
+
+                    // 3. boolean type
+                    json_str =  R"(true)";
+                    j = json::parse(json_str);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, true);
+
+                    json_str =  R"(false)";
+                    j = json::parse(json_str);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, false);
+
+                    // 4. null type
+                    json_str =  R"(null)";
+                    j = json::parse(json_str);
+                    validate_generated_json_and_start_end_pos_helper(json_str, j, nullptr);
+                }
+            }
+
+            SECTION("string type")
+            {
+                const std::string nested_type_json_str =  R"("test")";
+                const std::string root_type_json_str =  R"({ "a": 1,   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
+                auto expected = json({{"nested", "test"}, {"anotherValue", "test"}, {"a", 1}});
+                auto filteredExpected = expected;
+                filteredExpected.erase("a");
+                SETUP_TESTCASES()
+            }
+
+            SECTION("number type")
+            {
+                const std::string nested_type_json_str =  R"(2)";
+                const std::string root_type_json_str =  R"({ "a": 1,   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
+                auto expected = json({{"nested", 2}, {"anotherValue", "test"}, {"a", 1}});
+                auto filteredExpected = expected;
+                filteredExpected.erase("a");
+                SETUP_TESTCASES()
+            }
+
+            SECTION("boolean type")
+            {
+                const std::string nested_type_json_str =  R"(true)";
+                const std::string root_type_json_str =  R"({ "a": 1,   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
+                auto expected = json({{"nested", true}, {"anotherValue", "test"}, {"a", 1}});
+                auto filteredExpected = expected;
+                filteredExpected.erase("a");
+                SETUP_TESTCASES()
+            }
+
+            SECTION("null type")
+            {
+                const std::string nested_type_json_str =  R"(null)";
+                const std::string root_type_json_str =  R"({ "a": 1,   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
+                auto expected = json({{"nested", nullptr}, {"anotherValue", "test"}, {"a", 1}});
+                auto filteredExpected = expected;
+                filteredExpected.erase("a");
+                SETUP_TESTCASES()
+            }
+        }
+        SECTION("with leading whitespace and newlines around root JSON")
+        {
+            const std::string initial_whitespace = R"(
+                
+            )";
+            const std::string nested_type_json_str = R"({
+                "a": 1,
+                "nested": {
+                    "b": "test"
+                },
+                "anotherValue": "test"
+            })";
+            const std::string end_whitespace = R"(
+                
+            )";
+            const std::string root_type_json_str = initial_whitespace + nested_type_json_str + end_whitespace;
+
+            auto expected = json({{"a", 1}, {"nested", {{"b", "test"}}}, {"anotherValue", "test"}});
+
+            auto j = json::parse(root_type_json_str);
+
+            // 2. Check if the generated JSON is as expected
+            CHECK(j == expected);
+
+            // 3. Check if the start and end positions do not include the surrounding whitespace
+            CHECK(j.start_pos() == initial_whitespace.size());
+            CHECK(j.end_pos() == root_type_json_str.size() - end_whitespace.size());
+        }
+    }
+#undef SETUP_TESTCASES
+#endif
 }
 
 // this test relies on parse errors being thrown, so it is skipped when
@@ -1887,3 +2155,315 @@ TEST_CASE("last-read diagnostics are identical across input adapters")
     }
 }
 #endif // !defined(JSON_NOEXCEPTION)
+
+// this test characterizes the current (documented-by-example, not otherwise
+// specified) behavior of JSON_DIAGNOSTIC_POSITIONS positions with respect to
+// value lifetime (copy/move/swap/mutation), the various input adapters, and
+// user-driven SAX usage. It is regression protection, not a behavior
+// specification: if any of these checks fail after a change to json.hpp,
+// that change deliberately altered observable behavior and the test (and
+// this comment) should be updated accordingly, rather than "fixed" blindly.
+#if JSON_DIAGNOSTIC_POSITIONS
+TEST_CASE("diagnostic positions: value lifetime, input adapters, and SAX")
+{
+    SECTION("value lifetime")
+    {
+        SECTION("copy constructor copies positions, recursively")
+        {
+            // basic_json(const basic_json&) (json.hpp, around line 1192) copies
+            // start_position/end_position for the value itself; nested values
+            // are copied via their own copy constructor (through the copied
+            // object/array container), so positions are preserved throughout
+            // the whole tree.
+            const std::string s = R"({"a":1,"b":[1,2,3]})";
+            const json a = json::parse(s);
+            const json b = a; // NOLINT(performance-unnecessary-copy-initialization)
+
+            CHECK(b.start_pos() == a.start_pos());
+            CHECK(b.end_pos() == a.end_pos());
+            CHECK(b["b"].start_pos() == a["b"].start_pos());
+            CHECK(b["b"].end_pos() == a["b"].end_pos());
+            CHECK(b["b"][0].start_pos() == a["b"][0].start_pos());
+            CHECK(b["b"][0].end_pos() == a["b"][0].end_pos());
+
+            // sanity: the positions are meaningful (not all npos)
+            CHECK(b.start_pos() == 0);
+            CHECK(b.end_pos() == s.size());
+        }
+
+        SECTION("move constructor resets the moved-from value to npos")
+        {
+            // basic_json(basic_json&&) (json.hpp, around line 1265) copies
+            // other's start_position/end_position into *this and then resets
+            // other's to npos (see the "// cppcheck-suppress[accessForwarded]
+            // TODO check" comments there). Only the top-level moved-from value
+            // is affected; its (moved-away) children are gone along with it.
+            const std::string s = R"({"a":1,"b":[1,2,3]})";
+            json a = json::parse(s);
+            const auto a_start = a.start_pos();
+            const auto a_end = a.end_pos();
+            const auto nested_start = a["b"].start_pos();
+            const auto nested_end = a["b"].end_pos();
+
+            const json b(std::move(a));
+
+            // the destination retains the original positions, recursively
+            CHECK(b.start_pos() == a_start);
+            CHECK(b.end_pos() == a_end);
+            CHECK(b["b"].start_pos() == nested_start);
+            CHECK(b["b"].end_pos() == nested_end);
+
+            // the moved-from value is reset to a null and reports npos
+            CHECK(a.is_null()); // NOLINT(bugprone-use-after-move,clang-analyzer-cplusplus.Move)
+            CHECK(a.start_pos() == std::string::npos); // NOLINT(bugprone-use-after-move,clang-analyzer-cplusplus.Move)
+            CHECK(a.end_pos() == std::string::npos); // NOLINT(bugprone-use-after-move,clang-analyzer-cplusplus.Move)
+        }
+
+        SECTION("swap() does NOT exchange positions (likely a real bug, see below)")
+        {
+            // NOTE (characterizing, not fixing, for #5420): basic_json::swap()
+            // (json.hpp, around line 3540, and the friend swap() that forwards
+            // to it) swaps m_data.m_type and m_data.m_value but -- unlike
+            // copy-assignment's operator=(basic_json) (json.hpp, around line
+            // 1291), which swaps start_position/end_position as part of its
+            // copy-and-swap implementation -- it never touches
+            // start_position/end_position. So after swap(a, b), the *values*
+            // of a and b are exchanged, but their *positions* are not: each
+            // ends up with its own original position describing the other's
+            // new content. This looks like an oversight/inconsistency rather
+            // than intended behavior, and is flagged to the maintainer; this
+            // test only pins the current (surprising) behavior so a fix (or a
+            // deliberate decision to keep it) shows up here as an intentional
+            // change rather than a silent regression.
+            json a = json::parse(R"({"a":1})");
+            json b = json::parse(R"([1,2,3,4,5])");
+            const auto a_start = a.start_pos();
+            const auto a_end = a.end_pos();
+            const auto b_start = b.start_pos();
+            const auto b_end = b.end_pos();
+            // both start at 0 (root values start right away), but their
+            // lengths (and thus end positions) differ, which is enough to
+            // tell after the swap whether positions actually moved with
+            // the values
+            CHECK(a_end != b_end);
+
+            using std::swap;
+            swap(a, b);
+
+            // values were exchanged as expected ...
+            CHECK(a == json::parse(R"([1,2,3,4,5])"));
+            CHECK(b == json::parse(R"({"a":1})"));
+
+            // ... but positions were NOT: each variable kept its own
+            // original position, now describing the other's content
+            CHECK(a.start_pos() == a_start);
+            CHECK(a.end_pos() == a_end);
+            CHECK(b.start_pos() == b_start);
+            CHECK(b.end_pos() == b_end);
+        }
+
+        SECTION("mutating a parsed document leaves positions of unrelated values untouched")
+        {
+            // Positions are recorded once, during parsing, and are not
+            // recomputed on mutation. As a consequence, after a mutation the
+            // parent's own recorded span may no longer describe its current
+            // (serialized) content -- it still describes what was originally
+            // parsed. This is characterized here as current behavior, not
+            // asserted to be desirable or specified.
+            SECTION("operator[] adding a new object key")
+            {
+                const std::string s = R"({"a":1})";
+                json j = json::parse(s);
+                const auto root_start = j.start_pos();
+                const auto root_end = j.end_pos();
+                const auto a_start = j["a"].start_pos();
+                const auto a_end = j["a"].end_pos();
+
+                j["c"] = 42;
+
+                // the newly-added value was never parsed, so it has no position
+                CHECK(j["c"].start_pos() == std::string::npos);
+                CHECK(j["c"].end_pos() == std::string::npos);
+
+                // the existing sibling's position is unaffected
+                CHECK(j["a"].start_pos() == a_start);
+                CHECK(j["a"].end_pos() == a_end);
+
+                // the parent's own recorded span is left as-is (now stale:
+                // it still reflects the original, shorter `{"a":1}` string)
+                CHECK(j.start_pos() == root_start);
+                CHECK(j.end_pos() == root_end);
+            }
+
+            SECTION("push_back on a parsed array")
+            {
+                const std::string s = R"([1,2,3])";
+                json j = json::parse(s);
+                const auto root_start = j.start_pos();
+                const auto root_end = j.end_pos();
+                const auto first_start = j[0].start_pos();
+
+                j.push_back(4);
+
+                CHECK(j.back().start_pos() == std::string::npos);
+                CHECK(j.back().end_pos() == std::string::npos);
+                CHECK(j[0].start_pos() == first_start);
+                CHECK(j.start_pos() == root_start);
+                CHECK(j.end_pos() == root_end);
+            }
+
+            SECTION("erase on a parsed array shifts elements but keeps their own positions")
+            {
+                const std::string s = R"([1,2,3])";
+                json j = json::parse(s);
+                const auto second_start = j[1].start_pos();
+                const auto third_start = j[2].start_pos();
+                const auto root_start = j.start_pos();
+                const auto root_end = j.end_pos();
+
+                j.erase(0);
+
+                // remaining elements moved down an index, but each one still
+                // reports the position it had *before* the erase (i.e. its
+                // position in the original source string, not a
+                // recalculated one)
+                CHECK(j[0].start_pos() == second_start);
+                CHECK(j[1].start_pos() == third_start);
+
+                // the parent's own recorded span is again left as-is
+                CHECK(j.start_pos() == root_start);
+                CHECK(j.end_pos() == root_end);
+            }
+        }
+    }
+
+    SECTION("input adapters")
+    {
+        SECTION("wide string input: positions count transcoded UTF-8 bytes, not wide characters")
+        {
+            // 'é' (U+00E9) is a single code unit in a wchar_t/UTF-16 string, but
+            // transcodes to 2 bytes in UTF-8; the lexer only ever sees the
+            // transcoded UTF-8 byte stream, so reported positions are byte
+            // offsets into that UTF-8 stream, not indices into the original
+            // std::wstring.
+            const std::wstring ws = L"{\"a\":\"éé\"}";
+            CHECK(ws.size() == 10); // 10 wide characters
+
+            const json j = json::parse(ws);
+            CHECK(j.start_pos() == 0);
+            // the transcoded UTF-8 form is 2 bytes longer than the wide string,
+            // because each of the two 'é' characters becomes 2 UTF-8 bytes
+            CHECK(j.end_pos() == 12);
+            CHECK(j.end_pos() != ws.size());
+
+            const json& a = j["a"];
+            CHECK(a.start_pos() == 5);
+            CHECK(a.end_pos() == 11);
+        }
+
+        SECTION("BOM-prefixed input: start_pos() reflects the skipped 3-byte BOM")
+        {
+            const std::string s = "\xEF\xBB\xBF{\"a\":1}";
+            const json j = json::parse(s);
+
+            // the lexer silently skips the BOM before parsing the value, so
+            // the root value's recorded span starts right after it
+            CHECK(j.start_pos() == 3);
+            CHECK(j.end_pos() == s.size());
+        }
+
+        SECTION("std::istringstream: positions are consistent, not npos")
+        {
+            const std::string s = R"({"a":1,"b":2})";
+            std::istringstream ss(s);
+            const json j = json::parse(ss);
+
+            CHECK(j.start_pos() == 0);
+            CHECK(j.end_pos() == s.size());
+            CHECK(j["a"].start_pos() == 5);
+        }
+
+        SECTION("std::ifstream: positions are consistent, not npos")
+        {
+            const std::string s = R"({"a":1,"b":2})";
+            {
+                std::ofstream file("unit-class_parser_diagnostic_positions.tmp");
+                file << s;
+            }
+
+            {
+                std::ifstream f("unit-class_parser_diagnostic_positions.tmp");
+                const json j = json::parse(f);
+
+                CHECK(j.start_pos() == 0);
+                CHECK(j.end_pos() == s.size());
+                CHECK(j["a"].start_pos() == 5);
+            }
+
+            static_cast<void>(std::remove("unit-class_parser_diagnostic_positions.tmp"));
+        }
+
+        SECTION("iterator-pair input: positions are consistent, not npos")
+        {
+            const std::string s = R"({"a":1,"b":2})";
+            const json j = json::parse(s.begin(), s.end());
+
+            CHECK(j.start_pos() == 0);
+            CHECK(j.end_pos() == s.size());
+            CHECK(j["a"].start_pos() == 5);
+        }
+
+        SECTION("binary formats have no text positions")
+        {
+            // binary formats (CBOR, MessagePack, UBJSON, BSON, BJData) are
+            // parsed via detail::binary_reader, which never sets
+            // start_position/end_position on the values it produces (they
+            // have no notion of a text offset), so every value's position
+            // stays at its default of npos.
+            const json src = json::parse(R"({"a":1,"b":[1,2]})");
+
+            const json from_cbor = json::from_cbor(json::to_cbor(src));
+            CHECK(from_cbor.start_pos() == std::string::npos);
+            CHECK(from_cbor.end_pos() == std::string::npos);
+            CHECK(from_cbor["a"].start_pos() == std::string::npos);
+            CHECK(from_cbor["b"][0].start_pos() == std::string::npos);
+
+            const json from_msgpack = json::from_msgpack(json::to_msgpack(src));
+            CHECK(from_msgpack.start_pos() == std::string::npos);
+            CHECK(from_msgpack.end_pos() == std::string::npos);
+
+            const json from_ubjson = json::from_ubjson(json::to_ubjson(src));
+            CHECK(from_ubjson.start_pos() == std::string::npos);
+            CHECK(from_ubjson.end_pos() == std::string::npos);
+
+            const json from_bson_val = json::from_bson(json::to_bson(src));
+            CHECK(from_bson_val.start_pos() == std::string::npos);
+            CHECK(from_bson_val.end_pos() == std::string::npos);
+        }
+    }
+
+    SECTION("user-driven SAX consumers with no lexer report npos")
+    {
+        // json::parse() internally wires up its json_sax_dom_parser with a
+        // pointer to its own lexer (see parser.hpp), which is how positions
+        // get set at all. A user who constructs a json_sax_dom_parser
+        // directly (e.g. to drive it via json::sax_parse()) and does not
+        // supply a lexer pointer gets a consumer with m_lexer_ref == nullptr;
+        // every "if (m_lexer_ref)" guard in json_sax.hpp is then skipped, so
+        // every value it produces keeps its default, unset position (npos).
+        // This was previously true but silently unasserted (operator==
+        // ignores positions), see #5420.
+        json result;
+        nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> sdp(result);
+        const std::string s = R"({"a":1,"b":[1,2,3]})";
+        CHECK(json::sax_parse(s, &sdp));
+
+        CHECK(result.start_pos() == std::string::npos);
+        CHECK(result.end_pos() == std::string::npos);
+        CHECK(result["a"].start_pos() == std::string::npos);
+        CHECK(result["a"].end_pos() == std::string::npos);
+        CHECK(result["b"][0].start_pos() == std::string::npos);
+        CHECK(result["b"][0].end_pos() == std::string::npos);
+    }
+}
+#endif

tests/src/unit-diagnostic-positions-only.cpp

@@ -1,44 +0,0 @@
-//     __ _____ _____ _____
-//  __|  |   __|     |   | |  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"
-
-#ifdef JSON_DIAGNOSTICS
-    #undef JSON_DIAGNOSTICS
-#endif
-
-#define JSON_DIAGNOSTICS 0
-#define JSON_DIAGNOSTIC_POSITIONS 1
-#include <nlohmann/json.hpp>
-
-using json = nlohmann::json;
-
-TEST_CASE("Better diagnostics with positions only")
-{
-    SECTION("invalid type")
-    {
-        const std::string json_invalid_string = R"(
-        {
-            "address": {
-                "street": "Fake Street",
-                "housenumber": "1"
-            }
-        }
-        )";
-        json j = json::parse(json_invalid_string);
-        CHECK_THROWS_WITH_AS(j.at("address").at("housenumber").get<int>(),
-                             "[json.exception.type_error.302] (bytes 108-111) type must be number, but is string", json::type_error);
-    }
-
-    SECTION("invalid type without positions")
-    {
-        const json j = "foo";
-        CHECK_THROWS_WITH_AS(j.get<int>(),
-                             "[json.exception.type_error.302] type must be number, but is string", json::type_error);
-    }
-}

tests/src/unit-diagnostic-positions.cpp

@@ -8,7 +8,9 @@
 
 #include "doctest_compatibility.h"
 
-#define JSON_DIAGNOSTICS 1
+#ifndef JSON_DIAGNOSTICS
+    #define JSON_DIAGNOSTICS 1
+#endif
 #define JSON_DIAGNOSTIC_POSITIONS 1
 #include <nlohmann/json.hpp>
 
@@ -27,8 +29,13 @@ TEST_CASE("Better diagnostics with positions")
         }
         )";
         json j = json::parse(json_invalid_string);
+#if JSON_DIAGNOSTICS
         CHECK_THROWS_WITH_AS(j.at("address").at("housenumber").get<int>(),
                              "[json.exception.type_error.302] (/address/housenumber) (bytes 108-111) type must be number, but is string", json::type_error);
+#else
+        CHECK_THROWS_WITH_AS(j.at("address").at("housenumber").get<int>(),
+                             "[json.exception.type_error.302] (bytes 108-111) type must be number, but is string", json::type_error);
+#endif
     }
 
     SECTION("invalid type without positions")
@@ -74,7 +81,12 @@ TEST_CASE("Better diagnostics with positions")
         // (/foo/bar); the position of that parent is reported in the message
         const json doc = json::parse(R"({"foo":{"bar":"a string"}})");
         const json patch = json::parse(R"([{"op":"add","path":"/foo/bar/baz","value":1}])");
+#if JSON_DIAGNOSTICS
         CHECK_THROWS_WITH_AS(doc.patch(patch),
                              "[json.exception.out_of_range.411] (/foo/bar) (bytes 14-24) cannot add value: the JSON Patch 'add' target's parent is of type string, but must be an object or array", json::out_of_range);
+#else
+        CHECK_THROWS_WITH_AS(doc.patch(patch),
+                             "[json.exception.out_of_range.411] (bytes 14-24) cannot add value: the JSON Patch 'add' target's parent is of type string, but must be an object or array", json::out_of_range);
+#endif
     }
 }

tests/src/unit-no-macro-leak.cpp

@@ -1,34 +0,0 @@
-//     __ _____ _____ _____
-//  __|  |   __|     |   | |  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
-
-// This file makes sure that none of the internal JSON_HEDLEY_* macros (vendored
-// from https://nemequ.github.io/hedley/, see
-// include/nlohmann/thirdparty/hedley/hedley.hpp) leak into the including
-// translation unit. include/nlohmann/detail/macro_unscope.hpp is supposed to
-// #undef every JSON_HEDLEY_* macro (via hedley_undef.hpp) once json.hpp has
-// been fully processed. See https://github.com/nlohmann/json/issues/5408,
-// where JSON_HEDLEY_PRAGMA, JSON_HEDLEY_PREDICT_TRUE, JSON_HEDLEY_PREDICT_FALSE,
-// and JSON_HEDLEY_CLANG_HAS_DECLSPEC_ATTRIBUTE escaped this cleanup because
-// hedley_undef.hpp had no matching #undef for them.
-//
-// hedley_undef_checks.inc (included below) is generated at CMake configure/
-// build time by cmake/scripts/gen_hedley_undef_check.cmake, which derives the
-// full list of JSON_HEDLEY_* macro names directly from hedley.hpp. That way
-// this test covers every macro Hedley actually defines -- not a hardcoded
-// snapshot that would silently go stale the next time `make update_hedley`
-// runs -- and can never drift from the vendored header.
-
-#include "doctest_compatibility.h"
-
-#include <nlohmann/json.hpp>
-
-TEST_CASE("JSON_HEDLEY macros do not leak after including json.hpp")
-{
-#include "hedley_undef_checks.inc"
-    CHECK(true); // keep an assertion when nothing leaked
-}