diff --git a/libcxx/include/ciso646 b/libcxx/include/ciso646
--- a/libcxx/include/ciso646
+++ b/libcxx/include/ciso646
@@ -17,8 +17,12 @@

 #include <__config>

+#if _LIBCPP_STD_VER > 14
+#error "Including <ciso646> is forbidden since C++17"
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif

 #endif  // _LIBCPP_CISO646