Add scalar-on-left overloads for legacy discarded comparisons in C++20 - #5682
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With JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1 and C++20, a scalar on the left-hand side of <= or >= (e.g., `1 <= discarded`) yielded false instead of the documented true. The C++20 legacy block only had member operators, which are only candidates when the basic_json is the left operand; for a scalar on the left, overload resolution picked the candidate rewritten from operator<=>, which does not emulate the legacy behavior. The C++17 branch already has scalar-on-the-left friend overloads for <= and >=; add the equivalent pair to the C++20 legacy block. Added a regression test to tests/src/unit-comparison.cpp covering all four operand orders for both operators. Fixes #5665. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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- binary_reader: rename the error_handler constructor parameter, which shadowed the member (-Wshadow, -Wshadow-field-in-constructor; #5746) - basic_json(copy_construct_tag, ...): declare it noexcept when copying the base class is (GCC 16 -Wnoexcept; #5690) - the scalar-on-left legacy comparison operators: noexcept only when converting the scalar is, like their member counterparts (#5682, #5751) - compare_leaves: use std::is_eq/is_lt/is_gt instead of comparing a std::partial_ordering with 0 (-Wzero-as-null-pointer-constant; #5686) - serializer: silence MSVC C4127 for the EnsureAscii template parameter (#5741, #5746) - clang-tidy: return the sanitized reference in binary_writer, take the key of ordered_map::find_impl by const reference (#5727), and mark the switches over parse_array_index (#5728) - ordered_map: keep <memory> for std::allocator (IWYU) Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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* Keep the serializer conversion for objects whose keys cannot be converted #5591 added a test converting nlohmann::json into a basic_json whose string type cannot be constructed from std::string. That instantiates convert_iteratively(), whose members.emplace_back(next.key(), ...) needs exactly that key conversion, and broke the build of unit-alt-string. Dispatch on the key's constructibility and leave such conversions to the serializers, as the levels above the nesting bound already do (#3425). Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Fix the remaining CI failures on develop - unit-wstring: with a 16-bit wchar_t (Windows), a lone surrogate is reported as the ill-formed byte 0xFF since #5704; the std::wstring expectations still had the previous <U+0000>. - ci_single_binaries: json_literals.hpp (#5610) and json.hpp include each other on purpose, and IWYU, not following the cycle, asks to replace json.hpp with json_fwd.hpp. Report its findings without failing the build, as already done for json.hpp. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Fix the library warnings and noexcept specifications from the merged PRs - binary_reader: rename the error_handler constructor parameter, which shadowed the member (-Wshadow, -Wshadow-field-in-constructor; #5746) - basic_json(copy_construct_tag, ...): declare it noexcept when copying the base class is (GCC 16 -Wnoexcept; #5690) - the scalar-on-left legacy comparison operators: noexcept only when converting the scalar is, like their member counterparts (#5682, #5751) - compare_leaves: use std::is_eq/is_lt/is_gt instead of comparing a std::partial_ordering with 0 (-Wzero-as-null-pointer-constant; #5686) - serializer: silence MSVC C4127 for the EnsureAscii template parameter (#5741, #5746) - clang-tidy: return the sanitized reference in binary_writer, take the key of ordered_map::find_impl by const reference (#5727), and mark the switches over parse_array_index (#5728) - ordered_map: keep <memory> for std::allocator (IWYU) Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Split unit-conversions.cpp so MinGW can link it clang 18 with the MinGW linker failed to link test-conversions_cpp17 ("relocation truncated to fit: IMAGE_REL_AMD64_REL32"). As windows.yml recommends, keep the objects small by splitting the test file. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Fix the tests added by the merged PRs for all CI configurations - discard the results of dump() and from_*() in CHECK_THROWS with utils::ignore_return_value (GCC -Werror=unused-result) - give unit-bson's huge_string_t a default constructor (MSVC C2512, GCC 5, clang 3.5) - unit-disabled_exceptions: use the literals namespace when the global UDLs are off (ci_test_noglobaludls; #5700) - unit-binary_utf8_strict: expect the JSON pointer prefix with JSON_DIAGNOSTICS (#5741) - skip the tests that rely on exceptions under JSON_NOEXCEPTION (#5678, #5732) - clang-tidy and clang -Werror: static test data, CAPTURE(...);, const-correctness, use-after-move alias, unused conversion operator, a missing <iterator> include Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Title the macro examples and add JSON_STRICT_BINARY_UTF8 to the docset The documentation style check requires "Example: ..." titles on pages with several examples (#5741, #5591) and a docset entry for every macro page. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Regenerate BUILD.bazel and nlohmann_json.natvis Signed-off-by: Niels Lohmann <mail@nlohmann.me> #5746 added detail/output/error_handler.hpp and #5741 the json_abi_sbu8 ABI tag. * Install libidn11 for the CMake 3.5.0 binary in ci_cmake_flags Signed-off-by: Niels Lohmann <mail@nlohmann.me> #5733 moved ci_cmake_options from ubuntu:focal to ubuntu:24.04, which no longer ships libidn.so.11; the CMake 3.5.0 release binary links against it, so every ci_cmake_flags run has failed since. Install focal's libidn11 package for that matrix entry only. * Suppress Infer's false STACK_VARIABLE_ADDRESS_ESCAPE in get_impl get_impl() returns its local by value. A test added by the merged PRs instantiates it with a type Infer misreads, so ci_infer reported the 2021 code for the first time. Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Summary
With
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISONdefined to1and C++20,1 <= discardedand1 >= discardedreturnedfalseinstead of the documentedtruewhen the scalar is on theleft-hand side. The C++20 legacy block only defined member operators, which are candidates only
when the
basic_jsonis the left operand. For a scalar on the left, overload resolution insteadpicked the candidate rewritten from
operator<=>, which does not emulate the legacy behavior andyields
std::partial_ordering::unordered.Changes
friend bool operator<=(ScalarType, const_reference)andfriend bool operator>=(ScalarType, const_reference)to the C++20JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISONblock, matching the scalar-on-the-left overloadsthat already exist in the C++17 branch. A non-rewritten candidate wins over the rewritten one,
so there is no ambiguity.
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISONmacro page.Tests
tests/src/unit-comparison.cppregression test "regression Legacy discarded comparison in C++20: scalar <= discarded and scalar >= discarded yield false #5665 - scalar <= discardedand scalar >= discarded in C++20 legacy mode" (guarded by
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON), covering all four operand orders for<=and>=with a discarded value.tests/src/unit-comparison.cppagainstdevelop's headers (
633de8e44) with-std=c++20 -DJSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1;the four scalar-on-the-left checks failed exactly as in the issue's repro.
unit-comparison.cppwith clang++(
-Wall -Wextra -Werror -fsanitize=address,undefined -fno-sanitize-recover=undefined) for-std=c++11,-std=c++17, and-std=c++20, each with-DJSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1, and once more for-std=c++20without themacro (default mode). All configurations passed with 0 failed assertions.
Public API
No breaking changes. This only adds two new overloads inside the (deprecated, opt-in)
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1C++20 code path, bringing its scalar-on-the-leftbehavior in line with what is already documented and with the C++17 branch.
Fixes #5665
This PR was written by Claude Code on behalf of @nlohmann.
🤖 Generated with Claude Code