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 <jwakely@redhat.com>

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index 832dc6fd84ba0d192d7087254c3fe655e5248cfb..a616dc07b1070e060ed2df92e063cc34639e9723 100644 (file)
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1685,7 +1685,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _Tp, typename _Up>
     requires (!__is_derived_from_optional<_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<=> [[nodiscard]] (const optional<_Tp>& __x, const _Up& __v)
     { return bool(__x) ? *__x <=> __v : strong_ordering::less; }