[libc++] Deprecate removed features macros.

libcxx/docs/ReleaseNotes/18.rst

@@ -123,6 +123,12 @@ Deprecations and Removals
   overload of ``allocate``. However, this led to the library being non-conforming due to incorrect
   constexpr-ness.
 
+- The macros ``_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES`` and
+  ``_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES`` have been deprecated and
+  will be removed in LLVM 19. These macros used to enable all features
+  removed in C++17 and C++20. Instead of using these macros, use the
+  macros to enable the individual features.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 
@@ -149,6 +155,10 @@ LLVM 19
 - The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`` macro has been deprecated in LLVM 18 and will be removed
   entirely in LLVM 19.
 
+- The ``_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES`` and
+  ``_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES`` macros have been deprecated
+  in LLVM 18 and will be removed entirely in LLVM 19.
+
 LLVM 20
 ~~~~~~~
 

libcxx/docs/UsingLibcxx.rst

@@ -275,6 +275,8 @@ C++17 Specific Configuration Macros
 **_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES**:
   This macro is used to re-enable all the features removed in C++17. The effect
   is equivalent to manually defining each macro listed below.
+  This macro is deprecated and will be removed in LLVM-19. Use the
+  individual macros listed below.
 
 **_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR**:
   This macro is used to re-enable `auto_ptr`.
@@ -303,6 +305,8 @@ C++20 Specific Configuration Macros
 **_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES**:
   This macro is used to re-enable all the features removed in C++20. The effect
   is equivalent to manually defining each macro listed below.
+  This macro is deprecated and will be removed in LLVM-19. Use the
+  individual macros listed below.
 
 **_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS**:
   This macro is used to re-enable redundant members of `allocator<T>`,

libcxx/include/__config

@@ -16,6 +16,17 @@
 #  pragma GCC system_header
 #endif
 
+#if defined(_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES) && !defined(_LIBCPP_DISABLE_DEPRECATION_WARNINGS)
+#  pragma clang deprecated(                                                                                            \
+      _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES,                                                                           \
+      "_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES is deprecated in LLVM 18 and will be removed in LLVM 19")
+#endif
+#if defined(_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES) && !defined(_LIBCPP_DISABLE_DEPRECATION_WARNINGS)
+#  pragma clang deprecated(                                                                                            \
+      _LIBCPP_ENABLE_CXX20_REMOVED_FEATURES,                                                                           \
+      "_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES is deprecated in LLVM 18 and will be removed in LLVM 19")
+#endif
+
 #if defined(__apple_build_version__)
 // Given AppleClang XX.Y.Z, _LIBCPP_APPLE_CLANG_VER is XXYZ (e.g. AppleClang 14.0.3 => 1403)
 #  define _LIBCPP_COMPILER_CLANG_BASED

libcxx/test/libcxx/depr/enable_removed_cpp17_features.compile.pass.cpp

@@ -9,7 +9,7 @@
 // Test that defining _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES correctly defines
 // _LIBCPP_ENABLE_CXX17_REMOVED_FOO for each individual component macro.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES -Wno-deprecated-pragma
 
 #include <__config>
 

libcxx/test/libcxx/depr/enable_removed_cpp17_features.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
+//
+//===----------------------------------------------------------------------===//
+
+// <__config>
+
+// Ensure that defining _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES 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.
+
+// UNSUPPORTED: clang-modules-build
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES
+
+#include <__config> // expected-warning@* 1+ {{macro '_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES' has been marked as deprecated}}

libcxx/test/libcxx/depr/enable_removed_cpp20_features.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
+//
+//===----------------------------------------------------------------------===//
+
+// <__config>
+
+// Ensure that defining _LIBCPP_ENABLE_CXX20_REMOVED_FEATURES 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.
+
+// UNSUPPORTED: clang-modules-build
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES
+
+#include <version> // expected-warning@* 1+ {{macro '_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES' has been marked as deprecated}}