[gcc(refs/users/kmatsui/heads/is_const)] libstdc++: use new built-in trait __is_const

[gcc(refs/users/kmatsui/heads/is_const)] libstdc++: use new built-in trait __is_const

[Ken Matsui]

https://gcc.gnu.org/g:544f32e91cd20d8a66af8b71bba86e237269a6f4

commit 544f32e91cd20d8a66af8b71bba86e237269a6f4
Author: Ken Matsui <kmatsui@gcc.gnu.org>
Date:   Tue Mar 21 17:09:34 2023 -0700

    libstdc++: use new built-in trait __is_const
    
    This patch lets libstdc++ use new built-in trait __is_const.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/type_traits (is_const): Use __is_const built-in trait.
            (is_const_v): Likewise.
    
    Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>

Diff:
---
 libstdc++-v3/include/std/type_traits | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 0e7a9c9c7f3..3a46eca5377 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -764,6 +764,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Type properties.
 
   /// is_const
+#if __has_builtin(__is_const)
+  template<typename _Tp>
+    struct is_const
+    : public __bool_constant<__is_const(_Tp)>
+    { };
+#else
   template<typename>
     struct is_const
     : public false_type { };
@@ -771,6 +777,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp>
     struct is_const<_Tp const>
     : public true_type { };
+#endif
 
   /// is_volatile
   template<typename>
@@ -3210,10 +3217,17 @@ template <typename _Tp>
   inline constexpr bool is_compound_v = is_compound<_Tp>::value;
 template <typename _Tp>
   inline constexpr bool is_member_pointer_v = is_member_pointer<_Tp>::value;
+
+#if __has_builtin(__is_const)
+template <typename _Tp>
+  inline constexpr bool is_const_v = __is_const(_Tp);
+#else
 template <typename _Tp>
   inline constexpr bool is_const_v = false;
 template <typename _Tp>
   inline constexpr bool is_const_v<const _Tp> = true;
+#endif
+
 template <typename _Tp>
   inline constexpr bool is_volatile_v = false;
 template <typename _Tp>