[libc++] Deprecate and remove meaningless <cxxx> headers (llvm#111615)

This PR deprecates `<ccomplex>`, `<cstdbool>`, `<ctgmath>`, and
`<ciso646>` in C++17 and "removes" them in C++20 by special deprecation
warnings.

`<cstdalign>` is previously missing. This PR also tries to add them, and
then deprecates and "removes" `<cstdalign>`.

Papers:
- https://wg21.link/P0063R3
- https://wg21.link/P0619R4

Closes llvm#99985.

---------

Co-authored-by: Louis Dionne <[email protected]>

Author: 2 people

libcxx/docs/ReleaseNotes/20.rst

@@ -38,6 +38,7 @@ What's New in Libc++ 20.0.0?
 Implemented Papers
 ------------------
 
+- P0619R4: Reviewing Deprecated Facilities of C++17 for C++20 (`Github <https://github.com/llvm/llvm-project/issues/99985>`__)
 - P2747R2: ``constexpr`` placement new (`Github <https://github.com/llvm/llvm-project/issues/105427>`__)
 - P2609R3: Relaxing Ranges Just A Smidge (`Github <https://github.com/llvm/llvm-project/issues/105253>`__)
 - P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
@@ -95,6 +96,11 @@ Deprecations and Removals
   the ``_LIBCPP_VERBOSE_ABORT_NOT_NOEXCEPT`` macro can be defined to make the function non-``noexcept``. That macro
   will be removed in LLVM 21.
 
+- ``<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. The ``_LIBCPP_DISABLE_DEPRECATION_WARNINGS`` macro can be defined to
+  suppress deprecation for these headers.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 

libcxx/docs/Status/Cxx20Papers.csv

@@ -34,7 +34,7 @@
 "`P0528R3 <https://wg21.link/P0528R3>`__","The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange","2018-06 (Rapperswil)","","",""
 "`P0542R5 <https://wg21.link/P0542R5>`__","Support for contract based programming in C++","2018-06 (Rapperswil)","|Nothing To Do|","n/a","Pulled at the 2019-07 meeting in Cologne"
 "`P0556R3 <https://wg21.link/P0556R3>`__","Integral power-of-2 operations","2018-06 (Rapperswil)","|Complete|","9.0",""
-"`P0619R4 <https://wg21.link/P0619R4>`__","Reviewing Deprecated Facilities of C++17 for C++20","2018-06 (Rapperswil)","|Partial|","","Only sections D.7, D.8, D.9, D.10, D.11, D.12, and D.13 are implemented. Section D.4 remains undone."
+"`P0619R4 <https://wg21.link/P0619R4>`__","Reviewing Deprecated Facilities of C++17 for C++20","2018-06 (Rapperswil)","|Complete|","20.0","Removed headers are still provided as an extension, but with deprecation warnings"
 "`P0646R1 <https://wg21.link/P0646R1>`__","Improving the Return Value of Erase-Like Algorithms","2018-06 (Rapperswil)","|Complete|","10.0",""
 "`P0722R3 <https://wg21.link/P0722R3>`__","Efficient sized delete for variable sized classes","2018-06 (Rapperswil)","|Complete|","9.0",""
 "`P0758R1 <https://wg21.link/P0758R1>`__","Implicit conversion traits and utility functions","2018-06 (Rapperswil)","|Complete|","",""

libcxx/include/CMakeLists.txt

@@ -923,6 +923,7 @@ set(files
   coroutine
   csetjmp
   csignal
+  cstdalign
   cstdarg
   cstdbool
   cstddef

libcxx/include/ccomplex

@@ -23,4 +23,16 @@
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+
+using __standard_header_ccomplex _LIBCPP_DEPRECATED_("removed in C++20. Include <complex> instead.") = void;
+using __use_standard_header_ccomplex = __standard_header_ccomplex;
+
+#elif _LIBCPP_STD_VER >= 17
+
+using __standard_header_ccomplex _LIBCPP_DEPRECATED_("Include <complex> instead.") = void;
+using __use_standard_header_ccomplex                                               = __standard_header_ccomplex;
+
+#endif
+
 #endif // _LIBCPP_CCOMPLEX

libcxx/include/ciso646

@@ -21,4 +21,11 @@
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+
+using __standard_header_ciso646 _LIBCPP_DEPRECATED_("removed in C++20. Include <version> instead.") = void;
+using __use_standard_header_ciso646 = __standard_header_ciso646;
+
+#endif
+
 #endif // _LIBCPP_CISO646

libcxx/include/complex.h

@@ -24,7 +24,7 @@
 #endif
 
 #ifdef __cplusplus
-#  include <ccomplex>
+#  include <complex>
 #elif __has_include_next(<complex.h>)
 #  include_next <complex.h>
 #endif

libcxx/include/cstdalign

@@ -0,0 +1,55 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_CSTDALIGN
+#define _LIBCPP_CSTDALIGN
+
+/*
+    cstdalign synopsis
+
+Macros:
+
+    __alignas_is_defined
+    __alignof_is_defined
+
+*/
+
+#include <__config>
+
+// <stdalign.h> is not provided by libc++
+#if __has_include(<stdalign.h>)
+#  include <stdalign.h>
+#  ifdef _LIBCPP_STDALIGN_H
+#    error "If libc++ starts defining <stdalign.h>, the __has_include check should move to libc++'s <stdalign.h>"
+#  endif
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#undef __alignas_is_defined
+#define __alignas_is_defined 1
+
+#undef __alignof_is_defined
+#define __alignof_is_defined 1
+
+#if _LIBCPP_STD_VER >= 20
+
+using __standard_header_cstdalign _LIBCPP_DEPRECATED_("removed in C++20.") = void;
+using __use_standard_header_cstdalign                                      = __standard_header_cstdalign;
+
+#elif _LIBCPP_STD_VER >= 17
+
+using __standard_header_cstdalign _LIBCPP_DEPRECATED = void;
+using __use_standard_header_cstdalign                = __standard_header_cstdalign;
+
+#endif
+
+#endif // _LIBCPP_CSTDALIGN

libcxx/include/cstdbool

@@ -28,4 +28,16 @@ Macros:
 #undef __bool_true_false_are_defined
 #define __bool_true_false_are_defined 1
 
+#if _LIBCPP_STD_VER >= 20
+
+using __standard_header_cstdbool _LIBCPP_DEPRECATED_("removed in C++20.") = void;
+using __use_standard_header_cstdbool                                      = __standard_header_cstdbool;
+
+#elif _LIBCPP_STD_VER >= 17
+
+using __standard_header_cstdbool _LIBCPP_DEPRECATED = void;
+using __use_standard_header_cstdbool                = __standard_header_cstdbool;
+
+#endif
+
 #endif // _LIBCPP_CSTDBOOL

libcxx/include/ctgmath

@@ -18,11 +18,23 @@
 
 */
 
-#include <ccomplex>
 #include <cmath>
+#include <complex>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+
+using __standard_header_ctgmath _LIBCPP_DEPRECATED_("removed in C++20. Include <cmath> and <complex> instead.") = void;
+using __use_standard_header_ctgmath = __standard_header_ctgmath;
+
+#elif _LIBCPP_STD_VER >= 17
+
+using __standard_header_ctgmath _LIBCPP_DEPRECATED_("Include <cmath> and <complex> instead.") = void;
+using __use_standard_header_ctgmath = __standard_header_ctgmath;
+
+#endif
+
 #endif // _LIBCPP_CTGMATH

libcxx/include/module.modulemap

@@ -1109,6 +1109,11 @@ module std [system] {
     export *
   }
 
+  module cstdalign {
+    header "cstdalign"
+    export *
+  }
+
   module cstdarg {
     header "cstdarg"
     export *