[libc++] Deprecate the _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS macro

libcxx/docs/ReleaseNotes/18.rst

@@ -117,6 +117,12 @@ Deprecations and Removals
   and ``<experimental/vector>`` have been removed in LLVM 18, as all their contents will have been
   implemented in namespace ``std`` for at least two releases.
 
+- The macro ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`` has been deprecated and will be removed
+  in LLVM 19. This macro used to re-enable redundant members of ``std::allocator<T>`` like ``pointer``,
+  ``reference``, ``rebind``, ``address``, ``max_size``, ``construct``, ``destroy``, and the two-argument
+  overload of ``allocate``. However, this led to the library being non-conforming due to incorrect
+  constexpr-ness.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 
@@ -140,6 +146,9 @@ LLVM 19
 - The ``_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`` macro that changed the behavior for narrowing conversions
   in ``std::variant`` will be removed in LLVM 19.
 
+- The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`` macro has been deprecated in LLVM 18 and will be removed
+  entirely in LLVM 19.
+
 LLVM 20
 ~~~~~~~
 

libcxx/include/__memory/allocator.h

@@ -31,6 +31,12 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Tp>
 class allocator;
 
+#if defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS) && !defined(_LIBCPP_DISABLE_DEPRECATION_WARNINGS)
+#  pragma clang deprecated(                                                                                            \
+      _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS,                                                                  \
+      "_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS is deprecated in LLVM 18 and will be removed in LLVM 19")
+#endif
+
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION)
 // These specializations shouldn't be marked _LIBCPP_DEPRECATED_IN_CXX17.
 // Specializing allocator<void> is deprecated, but not using it.

libcxx/test/libcxx/depr/depr.default.allocator/allocator.members/address.depr_in_cxx17.verify.cpp

@@ -16,7 +16,7 @@
 
 // UNSUPPORTED: c++03, c++11, c++14
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS -Wno-deprecated-pragma
 
 #include <memory>
 

libcxx/test/libcxx/depr/depr.default.allocator/allocator.members/allocate.depr_in_cxx17.verify.cpp

@@ -15,7 +15,7 @@
 
 // UNSUPPORTED: c++03, c++11, c++14
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS -Wno-deprecated-pragma
 
 #include <memory>
 

libcxx/test/libcxx/depr/depr.default.allocator/enable_removed_allocator_members.deprecated.verify.cpp

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+
+// Ensure that defining _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS yields a
+// deprecation warning. We intend to issue a deprecation warning in LLVM 18
+// and remove the macro entirely in LLVM 19. As such, this test will be quite
+// short lived.
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS
+
+// UNSUPPORTED: clang-modules-build
+
+#include <memory> // expected-warning@* 1+ {{macro '_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS' has been marked as deprecated}}