[libc++] Deprecate and remove <cstdalign>

Author: frederick-vs-ja

libcxx/docs/ReleaseNotes/20.rst

@@ -78,9 +78,9 @@ Deprecations and Removals
   supported as an extension anymore, please migrate any code that uses e.g. ``std::vector<const T>`` to be
   standards conforming.
 
-- ``<ccomplex>``, ``<cstdbool>``, and ``<ctgmath>`` are deprecated since C++17 as specified by the standard. They,
-  together with ``<ciso646>``, are removed in C++20, but libc++ still provides these headers as an extension and only
-  deprecates them.
+- ``<ccomplex>``, ``<cstdalign>`` (previously missing), ``<cstdbool>``, and ``<ctgmath>`` are deprecated since C++17 as
+  specified by the standard. They, together with ``<ciso646>``, are removed in C++20, but libc++ still provides these
+  headers as an extension and only deprecates them.
 
 Upcoming Deprecations and Removals
 ----------------------------------

libcxx/docs/Status/Cxx17Papers.csv

@@ -43,7 +43,7 @@
 "","","","","",""
 "`P0032R3 <https://wg21.link/P0032R3>`__","Homogeneous interface for variant, any and optional","2016-06 (Oulu)","|Complete|","4.0",""
 "`P0040R3 <https://wg21.link/P0040R3>`__","Extending memory management tools","2016-06 (Oulu)","|Complete|","4.0",""
-"`P0063R3 <https://wg21.link/P0063R3>`__","C++17 should refer to C11 instead of C99","2016-06 (Oulu)","|Complete|","7.0","``<ccomplex>``, ``<cstdbool>``, and ``<ctgmath>`` are deprecated since LLVM 20."
+"`P0063R3 <https://wg21.link/P0063R3>`__","C++17 should refer to C11 instead of C99","2016-06 (Oulu)","|Complete|","7.0","``<ccomplex>``, ``<cstdalign>``, ``<cstdbool>``, and ``<ctgmath>`` are deprecated since LLVM 20."
 "`P0067R3 <https://wg21.link/P0067R3>`__","Elementary string conversions","2016-06 (Oulu)","|Nothing To Do|","n/a","Resolved by `P0067R5 <https://wg21.link/P0067R5>`__"
 "`P0083R3 <https://wg21.link/P0083R3>`__","Splicing Maps and Sets","2016-06 (Oulu)","|Complete|","8.0",""
 "`P0084R2 <https://wg21.link/P0084R2>`__","Emplace Return Type","2016-06 (Oulu)","|Complete|","4.0",""

libcxx/include/cstdalign

@@ -32,4 +32,17 @@ Macros:
 #undef __alignof_is_defined
 #define __alignof_is_defined 1
 
+#if _LIBCPP_STD_VER >= 17
+#  if _LIBCPP_STD_VER >= 20
+#    define _DEPRECATION_ANNOTATION _LIBCPP_DEPRECATED_("removed in C++20")
+#  else
+#    define _DEPRECATION_ANNOTATION _LIBCPP_DEPRECATED
+#  endif
+
+using __standard_header_cstdalign _DEPRECATION_ANNOTATION = void;
+using __use_standard_header_cstdalign                     = __standard_header_cstdalign;
+
+#  undef _DEPRECATION_ANNOTATION
+#endif
+
 #endif // _LIBCPP_CSTDALIGN

libcxx/test/std/depr/depr.cpp.headers/cstdalign.verify.cpp

@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <cstdalign>
+
+// check that <cstdalign> is deprecated in C++17 and removed in C++20
+
+// UNSUPPORTED: c++03, c++11, c++14
+
+#include "test_macros.h"
+
+#if TEST_STD_VER >= 20
+#  include <cstdalign> // expected-warning {{'__standard_header_cstdalign' is deprecated: removed in C++20}}
+#else
+#  include <cstdalign> // expected-warning {{'__standard_header_cstdalign' is deprecated}}
+#endif

libcxx/test/std/language.support/support.runtime/cstdalign.pass.cpp

@@ -6,7 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// test <cstdalign>
+// test <cstdalign> // deprecated in C++17, removed in C++20, but still provided by libc++ as an extension
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
 
 #include <cstdalign>