Index: libcxx/include/atomic
===================================================================
--- libcxx/include/atomic
+++ libcxx/include/atomic
@@ -2737,6 +2737,8 @@
 # define _LIBCPP_CONTENTION_LOCK_FREE false
 #endif
 
+#if _LIBCPP_STD_VER > 17
+
 #if ATOMIC_LLONG_LOCK_FREE == 2
 typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, long long>::type          __libcpp_signed_lock_free;
 typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, unsigned long long>::type __libcpp_unsigned_lock_free;
@@ -2751,10 +2753,15 @@
 typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, unsigned char>::type      __libcpp_unsigned_lock_free;
 #else
     // No signed/unsigned lock-free types
+# define _LIBCPP_CANNOT_DEFINE_ATOMIC_LOCK_FREE_TYPE_ALIASES
 #endif
 
+#ifndef _LIBCPP_CANNOT_DEFINE_ATOMIC_LOCK_FREE_TYPE_ALIASES
 typedef atomic<__libcpp_signed_lock_free> atomic_signed_lock_free;
 typedef atomic<__libcpp_unsigned_lock_free> atomic_unsigned_lock_free;
+#endif
+
+#endif // _LIBCPP_STD_VER > 17
 
 #define ATOMIC_FLAG_INIT {false}
 #define ATOMIC_VAR_INIT(__v) {__v}