[gcc r13-8194] libstdc++: Do not use __is_convertible unconditionally [PR113241]

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

https://gcc.gnu.org/g:4c66513656775d551db36b53c253cf236f0eeba8

commit r13-8194-g4c66513656775d551db36b53c253cf236f0eeba8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jan 5 12:03:22 2024 +0000

    libstdc++: Do not use __is_convertible unconditionally [PR113241]
    
    The new __is_convertible built-in should only be used after checking
    that it's supported.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/113241
            * include/std/type_traits (is_convertible_v): Guard use of
            built-in with preprocessor check.
    
    (cherry picked from commit 57fa5b60bbbf8038b8a699d2bcebd2a9b2e29aa4)

Diff:
---
 libstdc++-v3/include/std/type_traits | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 2bd607a8b8f..2b05e371953 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -3359,8 +3359,13 @@ template <typename _Tp>
 #endif
 template <typename _Base, typename _Derived>
   inline constexpr bool is_base_of_v = __is_base_of(_Base, _Derived);
+#if __has_builtin(__is_convertible)
 template <typename _From, typename _To>
   inline constexpr bool is_convertible_v = __is_convertible(_From, _To);
+#else
+template <typename _From, typename _To>
+  inline constexpr bool is_convertible_v = is_convertible<_From, _To>::value;
+#endif
 template<typename _Fn, typename... _Args>
   inline constexpr bool is_invocable_v = is_invocable<_Fn, _Args...>::value;
 template<typename _Fn, typename... _Args>