Add std::__is_byte<std::byte> specialization

	* include/bits/cpp_type_traits.h [__cplusplus >= 201703]
	(__is_byte<byte>): Define specialization for std::byte.

From-SVN: r262182

libstdc++-v3/ChangeLog

 2018-06-27  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/cpp_type_traits.h [__cplusplus >= 201703]
+	(__is_byte<byte>): Define specialization for std::byte.
+
 	PR libstdc++/86138
 	* include/bits/basic_string.tcc: [_GLIBCXX_EXTERN_TEMPLATE < 0]
 	Declare explicit instantiations of COW empty reps and I/O functions.

libstdc++-v3/include/bits/cpp_type_traits.h

@@ -391,6 +391,17 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
       typedef __true_type __type;
     };
 
+#if __cplusplus >= 201703L
+  enum class byte : unsigned char;
+
+  template<>
+    struct __is_byte<byte>
+    {
+      enum { __value = 1 };
+      typedef __true_type __type;
+    };
+#endif // C++17
+
   //
   // Move iterator type
   //