[libc++] Don't implement <stdatomic.h> before C++23 (#123130)

https://github.com/llvm/llvm-project/pull/95498 implemented a libc++
extension where <stdatomic.h> would forward to <atomic> even befor

[libc++] Don't implement <stdatomic.h> before C++23 (#123130)

https://github.com/llvm/llvm-project/pull/95498 implemented a libc++
extension where <stdatomic.h> would forward to <atomic> even before
C++23. Unfortunately, this was found to be a breaking change (with
fairly widespread impact) since that changes whether _Atomic(T) is a C
style atomic or std::atomic<T>. In principle, this can even be an ABI
break.

We generally don't implement extensions in libc++ because they cause so
many problems, and that extension had been accepted because it was
deemed pretty small and only a quality of life improvement. Since it has
widespread impact on valid C++20 (and before) code, this patch removes
the extension before we ship it in any public release.

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[libc++][C++03] Use `__cxx03/` headers in C++03 mode (#109002)

This patch implements the forwarding to frozen C++03 headers as
discussed in
https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-

[libc++][C++03] Use `__cxx03/` headers in C++03 mode (#109002)

This patch implements the forwarding to frozen C++03 headers as
discussed in
https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc. In the
RFC, we initially proposed selecting the right headers from the Clang
driver, however consensus seemed to steer towards handling this in the
library itself. This patch implements that direction.

At a high level, the changes basically amount to making each public
header look like this:

```
// inside <vector>
#ifdef _LIBCPP_CXX03_LANG
# include <__cxx03/vector>
#else
// normal <vector> content
#endif
```

In most cases, public headers are simple umbrella headers so there isn't
much code in the #else branch. In other cases, the #else branch contains
the actual implementation of the header.

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[libc++] Add #if 0 block to all the top-level headers (#119234)

Including The frozen C++03 headers results in a lot of formatting
changes in the main headers, so this splits these changes into a
sep

[libc++] Add #if 0 block to all the top-level headers (#119234)

Including The frozen C++03 headers results in a lot of formatting
changes in the main headers, so this splits these changes into a
separate commit instead.

This is part of
https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc.

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[libc++] Refactor the configuration macros to being always defined (#112094)

This is a follow-up to #89178. This updates the `<__config_site>`
macros.

[libc++][RFC] Always define internal feature test macros (#89178)

Currently, the library-internal feature test macros are only defined if
the feature is not available, and always have the prefix
`

[libc++][RFC] Always define internal feature test macros (#89178)

Currently, the library-internal feature test macros are only defined if
the feature is not available, and always have the prefix
`_LIBCPP_HAS_NO_`. This patch changes that, so that they are always
defined and have the prefix `_LIBCPP_HAS_` instead. This changes the
canonical use of these macros to `#if _LIBCPP_HAS_FEATURE`, which means
that using an undefined macro (e.g. due to a missing include) is
diagnosed now. While this is rather unlikely currently, a similar change
in `<__configuration/availability.h>` caught a few bugs. This also
improves readability, since it removes the double-negation of `#ifndef
_LIBCPP_HAS_NO_FEATURE`.

The current patch only touches the macros defined in `<__config>`. If
people are happy with this approach, I'll make a follow-up PR to also
change the macros defined in `<__config_site>`.

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[libc++] Enable C++ stdatomic.h for all C++ versions (#95498)

This extension is motivated by Android's use of libc++, where
<stdatomic.h> has redirected to <atomic> for many years, long before it

[libc++] Enable C++ stdatomic.h for all C++ versions (#95498)

This extension is motivated by Android's use of libc++, where
<stdatomic.h> has redirected to <atomic> for many years, long before it
was standardized in C++23.

When libc++'s stdatomic.h is included in C translation units, delegate
to the next stdatomic.h, which could come from Clang or libc.

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[libc++][stdatomic.h] Implements LWG3671. (#99775)

Implements:
- LWG3671 atomic_fetch_xor missing from stdatomic.h

[libc++] Format the code base (#74334)

This patch runs clang-format on all of libcxx/include and libcxx/src, in
accordance with the RFC discussed at [1]. Follow-up patches will format
the benchmar

[libc++] Format the code base (#74334)

This patch runs clang-format on all of libcxx/include and libcxx/src, in
accordance with the RFC discussed at [1]. Follow-up patches will format
the benchmarks, the test suite and remaining parts of the code. I'm
splitting this one into its own patch so the diff is a bit easier to
review.

This patch was generated with:

find libcxx/include libcxx/src -type f \
| grep -v 'module.modulemap.in' \
| grep -v 'CMakeLists.txt' \
| grep -v 'README.txt' \
| grep -v 'libcxx.imp' \
| grep -v '__config_site.in' \
| xargs clang-format -i

A Git merge driver is available in libcxx/utils/clang-format-merge-driver.sh
to help resolve merge and rebase issues across these formatting changes.

[1]: https://discourse.llvm.org/t/rfc-clang-formatting-all-of-libc-once-and-for-all

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[libc++][NFC] Replace _LIBCPP_STD_VER > x with _LIBCPP_STD_VER >= x

This change is almost fully mechanical. The only interesting change is in `generate_feature_test_macro_components.py` to generate

[libc++][NFC] Replace _LIBCPP_STD_VER > x with _LIBCPP_STD_VER >= x

This change is almost fully mechanical. The only interesting change is in `generate_feature_test_macro_components.py` to generate `_LIBCPP_STD_VER >=` instead. To avoid churn in the git-blame this commit should be added to the `.git-blame-ignore-revs` once committed.

Reviewed By: ldionne, var-const, #libc

Spies: jloser, libcxx-commits, arichardson, arphaman, wenlei

Differential Revision: https://reviews.llvm.org/D143962

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[libcxx] Make stdatomic.h work when included from a C source file

If a C source file includes the libc++ stdatomic.h, compilation will
break because (a) the C++ standard check will fail (which is ex

[libcxx] Make stdatomic.h work when included from a C source file

If a C source file includes the libc++ stdatomic.h, compilation will
break because (a) the C++ standard check will fail (which is expected),
and (b) `_LIBCPP_COMPILER_CLANG_BASED` won't be defined because the
logic defining it in `__config` is guarded by a `__cplusplus` check, so
we'll end up with a blank header. Move the detection logic outside of
the `__cplusplus` check to make the second check pass even in a C context
when you're using Clang. Note that `_LIBCPP_STD_VER` is not defined when
in C mode, hence stdatomic.h needs to check if in C++ mode before using
that macro to avoid a warning.

In an ideal world, a C source file wouldn't be including the libc++
header directory in its search path, so we'd never have this issue.
Unfortunately, certain build environments make this hard to guarantee,
and in this case it's easy to tweak this header to make it work in a C
context, so I'm hoping this is acceptable.

Fixes https://github.com/llvm/llvm-project/issues/57710.

Differential Revision: https://reviews.llvm.org/D134591

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[libc++] Remove overly conservative error in <stdatomic.h>

As mentionned in D97044, it is fine if users include <atomic> and then
include <stdatomic.h> -- we don't need to error out for that case.

[libc++] Remove overly conservative error in <stdatomic.h>

As mentionned in D97044, it is fine if users include <atomic> and then
include <stdatomic.h> -- we don't need to error out for that case.

Differential Revision: https://reviews.llvm.org/D125579

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[libc++][P0943] Add stdatomic.h header.

* https://wg21.link/P0943
* https://eel.is/c++draft/stdatomic.h.syn

This is a re-application of 5d1c1a24, which was reverted in 987c7f407
because it broke th

[libc++][P0943] Add stdatomic.h header.

* https://wg21.link/P0943
* https://eel.is/c++draft/stdatomic.h.syn

This is a re-application of 5d1c1a24, which was reverted in 987c7f407
because it broke the LLDB build.

Co-authored-by: Marek Kurdej <marek.kurdej@gmail.com>

Differential Revision: https://reviews.llvm.org/D97044

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[libc++] [C++2b] [P0943] Add stdatomic.h header.

* https://wg21.link/P0943
* https://eel.is/c++draft/stdatomic.h.syn

Differential Revision: https://reviews.llvm.org/D97044