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.

From-SVN: r276891

libstdc++-v3/ChangeLog

 2019-10-11  Jonathan Wakely  <jwakely@redhat.com>
 
+	* 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.
+
 	PR libstdc++/92059
 	* include/tr2/dynamic_bitset (__dynamic_bitset_base): Define all
 	special member functions as defaulted. Add noexcept to most members.

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>