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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>
This commit is contained in:
Niels Lohmann
2026-10-04 11:46:21 +02:00
committed by GitHub
parent d11e89471b
commit df27cc3d4c
4 changed files with 67 additions and 0 deletions
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tests/src/unit-comparison.cpp
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@@ -750,6 +750,28 @@ TEST_CASE("regression #3868 - heterogeneous comparisons compile under C++20 (P24
CHECK_FALSE(j != i);
}
}
#if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
TEST_CASE("regression #5665 - scalar <= discarded and scalar >= discarded in C++20 legacy mode")
{
// Issue #5665: with a scalar on the left-hand side, <= and >= only had the
// candidate rewritten from operator<=>, which does not emulate the legacy
// discarded-value behavior. Check that scalar-on-the-left now matches the
// other three operand orders.
const json discarded(json::value_t::discarded);
const json one = 1;
CHECK(discarded <= 1);
CHECK(discarded >= 1);
CHECK(one <= discarded);
CHECK(one >= discarded);
CHECK(1 <= discarded);
CHECK(1 >= discarded);
CHECK(1.5 <= discarded);
CHECK(1.5 >= discarded);
}
#endif
#endif
namespace