[PATCH] Use __is_same_as for std::is_same and std::is_same_v

[Jonathan Wakely]

By using the built-in we don't need to match a partial specialization
for std::is_same and don't need to instantiate std::is_same at all for
uses of std::is_same_v.

        * include/std/type_traits (is_same): Replace partial specialization
        by using __is_same_as built-in in primary template.
        (is_same_v): Use __is_same_as built-in instead of instantiating the
        is_same trait.

Tested x86_64-linux, committed to trunk.


commit ee208e530893aeb12b6a9edb1563a4e8ecaea887
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Sep 11 21:23:31 2019 +0100

    Use __is_same_as for std::is_same and std::is_same_v
    
    By using the built-in we don't need to match a partial specialization
    for std::is_same and don't need to instantiate std::is_same at all for
    uses of std::is_same_v.
    
            * include/std/type_traits (is_same): Replace partial specialization
            by using __is_same_as built-in in primary template.
            (is_same_v): Use __is_same_as built-in instead of instantiating the
            is_same trait.

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index dc8a019324d..4de5daa9f06 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -1388,13 +1388,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Type relations.
 
   /// is_same
-  template<typename, typename>
+  template<typename _Tp, typename _Up>
     struct is_same
-    : public false_type { };
-
-  template<typename _Tp>
-    struct is_same<_Tp, _Tp>
-    : public true_type { };
+    : public integral_constant<bool, __is_same_as(_Tp, _Up)>
+    { };
 
   /// is_base_of
   template<typename _Base, typename _Derived>
@@ -3158,7 +3155,7 @@ template <typename _Tp>
 template <typename _Tp, unsigned _Idx = 0>
   inline constexpr size_t extent_v = extent<_Tp, _Idx>::value;
 template <typename _Tp, typename _Up>
-  inline constexpr bool is_same_v = is_same<_Tp, _Up>::value;
+  inline constexpr bool is_same_v = __is_same_as(_Tp, _Up);
 template <typename _Base, typename _Derived>
   inline constexpr bool is_base_of_v = is_base_of<_Base, _Derived>::value;
 template <typename _From, typename _To>