libstdc++: Use variable template to fix -fconcepts-ts error [PR113366]

There's an error for -fconcepts-ts due to using a concept where a bool
NTTP is required, which is fixed by using the vraiable template that
already exists in the class scope.

This doesn't fix the problem with -fconcepts-ts as changes to the
placement of attributes is also needed.

libstdc++-v3/ChangeLog:

	PR testsuite/113366
	* include/std/format (basic_format_arg): Use __formattable
	variable template instead of __format::__formattable_with
	concept.

(cherry picked from commit 6c703b4eb68cbc32de1d62e5b573cb1b9857af29)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index d5ca962423cf1ddcd434d03cd1a420f44cd35d47..401503e895c4be76ea1ef74dcb97e3e96fe8711d 100644 (file)
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2847,8 +2847,7 @@ namespace __format
        // Format as const if possible, to reduce instantiations.
        template<typename _Tp>
          using __maybe_const_t
-           = __conditional_t<__format::__formattable_with<_Tp, _Context>,
-                             const _Tp, _Tp>;
+           = __conditional_t<__formattable<_Tp>, const _Tp, _Tp>;
 
        template<typename _Tq>
          static void
@@ -2866,7 +2865,7 @@ namespace __format
          explicit
          handle(_Tp& __val) noexcept
          {
-           if constexpr (!__format::__formattable_with<const _Tp, _Context>)
+           if constexpr (!__formattable<const _Tp>)
              static_assert(!is_const_v<_Tp>, "std::format argument must be "
                                              "non-const for this type");