[gcc r14-10044] libstdc++: Simplify constraints on <=> for std::reference_wrapper

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:d86472a6f041ccf3d1be0cf6bb15d1e0ad8f6dbe

commit r14-10044-gd86472a6f041ccf3d1be0cf6bb15d1e0ad8f6dbe
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Apr 19 17:42:04 2024 +0100

    libstdc++: Simplify constraints on <=> for std::reference_wrapper
    
    Instead of constraining these overloads in terms of synth-three-way we
    can just check that the value_type is less-than-comparable, which is
    what synth-three-way's constraints check.
    
    The reason that I implemented these with constraints has now been filed
    as LWG 4071, so add a comment about that too.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/refwrap.h (operator<=>): Simplify constraints.

Diff:
---
 libstdc++-v3/include/bits/refwrap.h | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/bits/refwrap.h b/libstdc++-v3/include/bits/refwrap.h
index fd1cc2b63e6..71ec2b297b7 100644
--- a/libstdc++-v3/include/bits/refwrap.h
+++ b/libstdc++-v3/include/bits/refwrap.h
@@ -384,23 +384,29 @@ _GLIBCXX_MEM_FN_TRAITS(&& noexcept, false_type, true_type)
 	&& requires { { __x.get() == __y.get() } -> convertible_to<bool>; }
       { return __x.get() == __y.get(); }
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 4071. reference_wrapper comparisons are not SFINAE-friendly
+
       [[nodiscard]]
       friend constexpr auto
-      operator<=>(reference_wrapper __x, reference_wrapper<_Tp> __y)
-      requires requires { __detail::__synth3way(__x.get(), __y.get()); }
+      operator<=>(reference_wrapper __x, reference_wrapper __y)
+      requires requires (const _Tp __t) {
+	{ __t < __t } -> __detail::__boolean_testable;
+      }
       { return __detail::__synth3way(__x.get(), __y.get()); }
 
       [[nodiscard]]
       friend constexpr auto
       operator<=>(reference_wrapper __x, const _Tp& __y)
-      requires requires { __detail::__synth3way(__x.get(), __y); }
+      requires requires { { __y < __y } -> __detail::__boolean_testable; }
       { return __detail::__synth3way(__x.get(), __y); }
 
       [[nodiscard]]
       friend constexpr auto
       operator<=>(reference_wrapper __x, reference_wrapper<const _Tp> __y)
-      requires (!is_const_v<_Tp>)
-	&& requires { __detail::__synth3way(__x.get(), __y.get()); }
+      requires (!is_const_v<_Tp>) && requires (const _Tp __t) {
+	{ __t < __t } -> __detail::__boolean_testable;
+      }
       { return __detail::__synth3way(__x.get(), __y.get()); }
 #endif
     };