diff --git a/libcxx/include/__memory/allocator.h b/libcxx/include/__memory/allocator.h
--- a/libcxx/include/__memory/allocator.h
+++ b/libcxx/include/__memory/allocator.h
@@ -262,10 +262,14 @@
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_SINCE_CXX20
 bool operator==(const allocator<_Tp>&, const allocator<_Up>&) _NOEXCEPT {return true;}
 
+#if _LIBCPP_STD_VER <= 17
+
 template <class _Tp, class _Up>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_SINCE_CXX20
 bool operator!=(const allocator<_Tp>&, const allocator<_Up>&) _NOEXCEPT {return false;}
 
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___MEMORY_ALLOCATOR_H
diff --git a/libcxx/include/memory b/libcxx/include/memory
--- a/libcxx/include/memory
+++ b/libcxx/include/memory
@@ -160,7 +160,7 @@
 bool operator==(const allocator<T>&, const allocator<U>&) noexcept; // constexpr in C++20
 
 template <class T, class U>
-bool operator!=(const allocator<T>&, const allocator<U>&) noexcept; // constexpr in C++20
+bool operator!=(const allocator<T>&, const allocator<U>&) noexcept; // removed in C++20
 
 template <class OutputIterator, class T>
 class raw_storage_iterator // deprecated in C++17, removed in C++20