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libstdc++: Implement LWG 3746 for std::optional

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This avoids constraint recursion in operator<=> for std::optional.
The resolution was approved in Kona 2022.

libstdc++-v3/ChangeLog:

	* include/std/optional (__is_derived_from_optional): New
	concept.
	(operator<=>): Use __is_derived_from_optional.
	* testsuite/20_util/optional/relops/lwg3746.cc: New test.
This commit is contained in:
Jonathan Wakely 2024-06-25 21:58:34 +01:00 committed by Jonathan Wakely
parent 952e67c0d1
commit c429d509a8
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2 changed files with 30 additions and 2 deletions
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12
libstdc++-v3/include/std/optional
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@ -1581,9 +1581,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ return !__rhs; }
#endif // three-way-comparison
#if __cpp_lib_concepts
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 4072. std::optional comparisons: constrain harder
#if __cpp_lib_concepts
# define _REQUIRES_NOT_OPTIONAL(T) requires (!__is_optional_v<T>)
#else
# define _REQUIRES_NOT_OPTIONAL(T)
@ -1675,8 +1675,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ return !__rhs || __lhs >= *__rhs; }
#ifdef __cpp_lib_three_way_comparison
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3746. optional's spaceship with U with a type derived from optional
// causes infinite constraint meta-recursion
template<typename _Tp>
concept __is_derived_from_optional = requires (const _Tp& __t) {
[]<typename _Up>(const optional<_Up>&){ }(__t);
};
template<typename _Tp, typename _Up>
requires (!__is_optional_v<_Up>)
requires (!__is_derived_from_optional<_Up>)
&& three_way_comparable_with<_Up, _Tp>
constexpr compare_three_way_result_t<_Tp, _Up>
operator<=> [[nodiscard]] (const optional<_Tp>& __x, const _Up& __v)

20
libstdc++-v3/testsuite/20_util/optional/relops/lwg3746.cc Normal file
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@ -0,0 +1,20 @@
// { dg-do compile { target c++20 } }
// LWG 3746. optional's spaceship with U with a type derived from optional
// causes infinite constraint meta-recursion
#include <optional>
struct S : std::optional<char>
{
bool operator==(const S&) const;
bool operator<(const S&) const;
bool operator>(const S&) const;
bool operator<=(const S&) const;
bool operator>=(const S&) const;
};
auto cmp(const S& s, const std::optional<char>& o)
{
return s <=> o;
}