Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -21,6 +21,12 @@
 #define _GNUC_VER 0
 #endif
 
+#ifdef __clang__
+#define _CLANG_VER (__clang_major__ * 100 + __clang_minor__)
+#else
+#define _CLANG_VER 0
+#endif
+
 #if !_WIN32
 #include <unistd.h>
 #endif
@@ -577,7 +583,11 @@
 #endif  // _LIBCPP_HAS_NO_STATIC_ASSERT
 
 #ifdef _LIBCPP_HAS_NO_DECLTYPE
-#define decltype(x) __typeof__(x)
+# if _CLANG_VER >= 304 || _GNUC_VER >= 406
+#   define decltype(__x) __decltype(__x)
+# else
+#   define decltype(__x) __typeof__(__x)
+# endif
 #endif
 
 #ifdef _LIBCPP_HAS_NO_CONSTEXPR