[libc++] Deprecate and remove meaningless <cxxx> headers (#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 #99985.

---------

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

Author: frederick-vs-ja

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>`__)
@@ -89,6 +90,11 @@ 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 *

libcxx/include/tgmath.h

@@ -24,7 +24,8 @@
 #endif
 
 #ifdef __cplusplus
-#  include <ctgmath>
+#  include <cmath>
+#  include <complex>
 #else
 #  if __has_include_next(<tgmath.h>)
 #    include_next <tgmath.h>

libcxx/test/libcxx/clang_modules_include.gen.py

@@ -17,10 +17,15 @@
 
 import sys
 sys.path.append(sys.argv[1])
-from libcxx.header_information import lit_header_restrictions, public_headers
+from libcxx.header_information import (
+    lit_header_restrictions,
+    lit_header_undeprecations,
+    public_headers,
+)
 
 for header in public_headers:
-  print(f"""\
+    print(
+        f"""\
 //--- {header}.compile.pass.cpp
 // RUN: %{{cxx}} %s %{{flags}} %{{compile_flags}} -fmodules -fcxx-modules -fmodules-cache-path=%t -fsyntax-only
 
@@ -41,9 +46,11 @@
 // UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
 
 {lit_header_restrictions.get(header, '')}
+{lit_header_undeprecations.get(header, '')}
 
 #include <{header}>
-""")
+"""
+    )
 
 print(
     f"""\

libcxx/test/libcxx/double_include.gen.py

@@ -15,12 +15,18 @@
 
 import sys
 sys.path.append(sys.argv[1])
-from libcxx.header_information import lit_header_restrictions, public_headers
+from libcxx.header_information import (
+    lit_header_restrictions,
+    lit_header_undeprecations,
+    public_headers,
+)
 
 for header in public_headers:
-  print(f"""\
+    print(
+        f"""\
 //--- {header}.sh.cpp
 {lit_header_restrictions.get(header, '')}
+{lit_header_undeprecations.get(header, '')}
 
 // RUN: %{{cxx}} -c %s -o %t.first.o %{{flags}} %{{compile_flags}}
 // RUN: %{{cxx}} -c %s -o %t.second.o -DWITH_MAIN %{{flags}} %{{compile_flags}}
@@ -32,4 +38,5 @@
 #if defined(WITH_MAIN)
 int main(int, char**) {{ return 0; }}
 #endif
-""")
+"""
+    )

libcxx/test/libcxx/header_inclusions.gen.py

@@ -12,32 +12,43 @@
 # RUN: %{python} %s %{libcxx-dir}/utils
 
 import sys
+
 sys.path.append(sys.argv[1])
-from libcxx.header_information import lit_header_restrictions, public_headers, mandatory_inclusions
+from libcxx.header_information import (
+    lit_header_restrictions,
+    lit_header_undeprecations,
+    public_headers,
+    mandatory_inclusions,
+)
 
 for header in public_headers:
-  header_guard = lambda h: f"_LIBCPP_{str(h).upper().replace('.', '_').replace('/', '_')}"
+    header_guard = (
+        lambda h: f"_LIBCPP_{str(h).upper().replace('.', '_').replace('/', '_')}"
+    )
 
-  # <cassert> has no header guards
-  if header == 'cassert':
-    checks = ''
-  else:
-    checks = f'''
+    # <cassert> has no header guards
+    if header == "cassert":
+        checks = ""
+    else:
+        checks = f"""
 #ifndef {header_guard(header)}
 # error <{header}> was expected to define a header guard {header_guard(header)}
 #endif
-'''
-  for includee in mandatory_inclusions.get(header, []):
-    checks += f'''
+"""
+    for includee in mandatory_inclusions.get(header, []):
+        checks += f"""
 #ifndef {header_guard(includee)}
 # error <{header}> was expected to include <{includee}>
 #endif
-'''
+"""
 
-  print(f"""\
+    print(
+        f"""\
 //--- {header}.compile.pass.cpp
 {lit_header_restrictions.get(header, '')}
+{lit_header_undeprecations.get(header, '')}
 
 #include <{header}>
 {checks}
-""")
+"""
+    )

libcxx/test/libcxx/include_as_c.sh.cpp

@@ -34,6 +34,7 @@
 #endif
 #include <math.h>
 #include <setjmp.h>
+#include <stdalign.h>
 #include <stdatomic.h>
 #include <stdbool.h>
 #include <stddef.h>

libcxx/test/libcxx/libcpp_version.gen.py

@@ -12,16 +12,23 @@
 
 import sys
 sys.path.append(sys.argv[1])
-from libcxx.header_information import lit_header_restrictions, public_headers
+from libcxx.header_information import (
+    lit_header_restrictions,
+    lit_header_undeprecations,
+    public_headers,
+)
 
 for header in public_headers:
-  print(f"""\
+    print(
+        f"""\
 //--- {header}.compile.pass.cpp
 {lit_header_restrictions.get(header, '')}
+{lit_header_undeprecations.get(header, '')}
 
 #include <{header}>
 
 #ifndef _LIBCPP_VERSION
 # error <{header}> does not seem to define _LIBCPP_VERSION
 #endif
-""")
+"""
+    )

libcxx/test/libcxx/no_assert_include.gen.py

@@ -12,20 +12,28 @@
 # RUN: %{python} %s %{libcxx-dir}/utils
 
 import sys
+
 sys.path.append(sys.argv[1])
-from libcxx.header_information import lit_header_restrictions, public_headers
+from libcxx.header_information import (
+    lit_header_restrictions,
+    lit_header_undeprecations,
+    public_headers,
+)
 
 for header in public_headers:
-  if header == 'cassert':
-    continue
+    if header == "cassert":
+        continue
 
-  print(f"""\
+    print(
+        f"""\
 //--- {header}.compile.pass.cpp
 {lit_header_restrictions.get(header, '')}
+{lit_header_undeprecations.get(header, '')}
 
 #include <{header}>
 
 #ifdef assert
 # error "Do not include cassert or assert.h in standard header files"
 #endif
-""")
+"""
+    )