[libc++] Use __is_convertible built-in when available

https://github.com/llvm/llvm-project/issues/62396 reports that
GCC 13 barfs on parsing <type_traits> because of the declarations
of `struct __is_convertible`.  In GCC 13, `__is_convertible` is a
built-in, but `__is_convertible_to` is not.  Clang has both, so
using either should be fine.

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D149313

diff --git a/libcxx/include/__type_traits/is_convertible.h b/libcxx/include/__type_traits/is_convertible.h
index 873a64b..749d6fd 100644 (file)
--- a/libcxx/include/__type_traits/is_convertible.h
+++ b/libcxx/include/__type_traits/is_convertible.h
@@ -24,11 +24,18 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
+#if __has_builtin(__is_convertible) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
+
+template <class _T1, class _T2>
+struct _LIBCPP_TEMPLATE_VIS is_convertible : public integral_constant<bool, __is_convertible(_T1, _T2)> {};
+
+#elif __has_builtin(__is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
 
 template <class _T1, class _T2> struct _LIBCPP_TEMPLATE_VIS is_convertible
     : public integral_constant<bool, __is_convertible_to(_T1, _T2)> {};
 
+// TODO: Remove this fallback when GCC < 13 support is no longer required.
+// GCC 13 has the __is_convertible built-in.
 #else  // __has_builtin(__is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
 
 namespace __is_convertible_imp