vec.h (vnull::operator vec): Add constexpr keyword for C++11 and later.

	* vec.h (vnull::operator vec): Add constexpr keyword for
	C++11 and later.

From-SVN: r240443

gcc/ChangeLog

+2016-09-23  Jakub Jelinek  <jakub@redhat.com>
+
+	* vec.h (vnull::operator vec): Add constexpr keyword for
+	C++11 and later.
+
 2016-09-23  Doug Gilmore  <doug.gilmore@imgtec.com>
 
 	PR tree-optimization/77654

gcc/vec.h

@@ -414,6 +414,9 @@ struct GTY((user)) vec
 struct vnull
 {
   template <typename T, typename A, typename L>
+#if __cpp_constexpr >= 200704
+  constexpr
+#endif
   operator vec<T, A, L> () { return vec<T, A, L>(); }
 };
 extern vnull vNULL;