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Add scalar-on-left overloads for legacy discarded comparisons in C++20

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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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Niels Lohmann
2026-09-29 23:16:44 +02:00
parent 633de8e44b
commit 864ac37cdb
4 changed files with 67 additions and 0 deletions
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docs/mkdocs/docs/api/macros/json_use_legacy_discarded_value_comparison.md
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@@ -79,3 +79,6 @@ When the macro is not defined, the library will define it to its default value.
## Version history
- Added in version 3.11.0.
- Fixed in version 3.13.0 so `<=` and `>=` also emulate the legacy behavior in C++20 when the JSON value is the
right-hand operand of a scalar comparison; before, only the 3-way-comparison-rewritten candidate was found, which
yielded `#!cpp false` instead of `#!cpp true`.