[PATCH] Define __cpp_lib_is_constant_evaluated

[Jakub Jelinek]

Hi!

Seems __cpp_lib_is_constant_evaluated should be defined to 201811L
if std::is_constant_evaluated() is implemented.

Ok for trunk?

2019-01-11  Jakub Jelinek  <ja...@redhat.com>

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

--- libstdc++-v3/include/std/type_traits.jj     2019-01-01 12:45:50.842546655 
+0100
+++ libstdc++-v3/include/std/type_traits        2019-01-11 16:44:39.546916488 
+0100
@@ -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.jj 2019-01-10 16:42:18.081062695 +0100
+++ libstdc++-v3/include/std/version    2019-01-11 16:48:35.944119391 +0100
@@ -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

        Jakub