type_traits (__cpp_lib_is_constant_evaluated): Define.

	* include/std/type_traits (__cpp_lib_is_constant_evaluated): Define.
	* include/std/version (__cpp_lib_is_constant_evaluated): Define.

From-SVN: r267847

libstdc++-v3/ChangeLog

+2019-01-11  Jakub Jelinek  <jakub@redhat.com>
+
+	* include/std/type_traits (__cpp_lib_is_constant_evaluated): Define.
+	* include/std/version (__cpp_lib_is_constant_evaluated): Define.
+
 2019-01-11  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/88802

libstdc++-v3/include/std/type_traits

@@ -3030,6 +3030,9 @@ template <typename _From, typename _To>
     using unwrap_ref_decay_t = typename unwrap_ref_decay<_Tp>::type;
 
 #ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+
+#define __cpp_lib_is_constant_evaluated 201811L 	
+
   constexpr inline bool
   is_constant_evaluated() noexcept
   { return __builtin_is_constant_evaluated(); }

libstdc++-v3/include/std/version

@@ -142,6 +142,9 @@
 
 #if __cplusplus > 201703L
 // c++2a
+#ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+# define __cpp_lib_is_constant_evaluated 201811L
+#endif
 #define __cpp_lib_list_remove_return_type 201806L
 #endif // C++2a
 #endif // C++17