From d23d35d3b76aa2ca124e580908f56f4b249cfe3a Mon Sep 17 00:00:00 2001 From: Patrick Palka Date: Fri, 28 Feb 2025 10:15:45 -0500 Subject: [PATCH] libstdc++: Improve optional's <=> constraint recursion workaround [PR104606] It turns out the reason the behavior of this testcase changed after CWG 2369 is because validity of the substituted return type is now checked later, after constraints. So a more reliable workaround for this issue is to add a constraint to check the validity of the return type earlier, matching the pre-CWG 2369 semantics. PR libstdc++/104606 libstdc++-v3/ChangeLog: * include/std/optional (operator<=>): Revert r14-9771 change. Add constraint checking the validity of the return type compare_three_way_result_t before the three_way_comparable_with constraint. Reviewed-by: Jonathan Wakely (cherry picked from commit 815f1f27a1dba2f0acd1f02d0beafedadebe967c) --- libstdc++-v3/include/std/optional | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional index 3507c36a4d8..1c37c8c5ac8 100644 --- a/libstdc++-v3/include/std/optional +++ b/libstdc++-v3/include/std/optional @@ -1430,7 +1430,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION #ifdef __cpp_lib_three_way_comparison template requires (!__is_optional_v<_Up>) - && three_way_comparable_with<_Up, _Tp> + && requires { typename compare_three_way_result_t<_Tp, _Up>; } + && three_way_comparable_with<_Tp, _Up> constexpr compare_three_way_result_t<_Tp, _Up> operator<=>(const optional<_Tp>& __x, const _Up& __v) { return bool(__x) ? *__x <=> __v : strong_ordering::less; } -- 2.47.2