[libc++][NFC] Add missing license headers

Also standardize the license comment in several files where it was
different from what we normally do.

[libc++][format] LWG4106: `basic_format_args` should not be default-constructible (#97250)

See [LWG4106](https://cplusplus.github.io/LWG/issue4106) and
[P3341R0](https://wg21.link/p3341r0).

The

[libc++][format] LWG4106: `basic_format_args` should not be default-constructible (#97250)

See [LWG4106](https://cplusplus.github.io/LWG/issue4106) and
[P3341R0](https://wg21.link/p3341r0).

The test coverage for the empty state of `basic_format_args` in
`get.pass.cpp` is to be completely removed, because the
non-default-constructibility is covered in `ctor.pass.cpp`.

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[libc++] Some tests are missing include for `numeric_limits` (#90345)

Noticed while attempting microsoft/STL#4634

Reland: [libc++][format] P2637R3: Member visit (std::basic_format_arg) #76449 (#79032)

Deleted the offending test case.


`libcxx/test/std/utilities/format/format.arguments/format.arg/visit.retur

Reland: [libc++][format] P2637R3: Member visit (std::basic_format_arg) #76449 (#79032)

Deleted the offending test case.


`libcxx/test/std/utilities/format/format.arguments/format.arg/visit.return_type.pass.cpp`
lines: 134-135:
> test<Context, bool, long>(true, 192812079084L);
test<Context, bool, long>(false, 192812079084L);

Relands: https://github.com/llvm/llvm-project/pull/76449
Reverted in:
https://github.com/llvm/llvm-project/commit/02f95b77515fe18ed1076b94cbb850ea0cf3c77e

---------

Co-authored-by: Zingam <zingam@outlook.com>

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Revert "[libc++][format] P2637R3: Member `visit` (`std::basic_format_arg`) (#76449)"

This reverts commit 7d9b5aa65b09126031e1c2903605a7d34aea4bc1 since
std/utilities/format/format.arguments/format.a

Revert "[libc++][format] P2637R3: Member `visit` (`std::basic_format_arg`) (#76449)"

This reverts commit 7d9b5aa65b09126031e1c2903605a7d34aea4bc1 since
std/utilities/format/format.arguments/format.arg/visit.return_type.pass.cpp
is failing on Windows when building with Clang-cl.

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[libc++][format] P2637R3: Member `visit` (`std::basic_format_arg`) (#76449)

Implements parts of: `P2637R3` https://wg21.link/P2637R3
(https://eel.is/c++draft/variant.visit)

Implements:
`basic_f

[libc++][format] P2637R3: Member `visit` (`std::basic_format_arg`) (#76449)

Implements parts of: `P2637R3` https://wg21.link/P2637R3
(https://eel.is/c++draft/variant.visit)

Implements:
`basic_format_arg.visit()`
`basic_format_arg.visit<R>()`
Deprecates:
`std::visit_format_arg()`

The tests are as close as possible to the non-member function tests.

To land after: https://github.com/llvm/llvm-project/pull/76447,
https://github.com/llvm/llvm-project/pull/76268

---------

Co-authored-by: Zingam <zingam@outlook.com>

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[libc++] Mark slow tests as unsupported on GCC

Some tests in our test suite are unbelievably slow on GCC due to the
use of the always_inline attribute. See [1] for more details.

This patch introduc

[libc++] Mark slow tests as unsupported on GCC

Some tests in our test suite are unbelievably slow on GCC due to the
use of the always_inline attribute. See [1] for more details.

This patch introduces the GCC-ALWAYS_INLINE-FIXME lit feature to
disable tests that are plagued by that issue. At the same time, it
moves several existing tests from ad-hoc `UNSUPPORTED: gcc-12` markup
to the new GCC-ALWAYS_INLINE-FIXME feature, and marks the slowest tests
reported by the CI as `UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME`.

[1]: https://discourse.llvm.org/t/rfc-stop-supporting-extern-instantiations-with-gcc/71277/1

Differential Revision: https://reviews.llvm.org/D152736

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[libc++][format] Removes the experimental status.

The code has been quite ready for a while now and there are no more ABI
breaking papers. So this is a good time to mark the feature as stable.

Revi

[libc++][format] Removes the experimental status.

The code has been quite ready for a while now and there are no more ABI
breaking papers. So this is a good time to mark the feature as stable.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D150802

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[libc++] Remove availability markup for std::format

std::format is currently experimental, so there is technically no
deployment target requirement for it (since the only symbols required
for it are

[libc++] Remove availability markup for std::format

std::format is currently experimental, so there is technically no
deployment target requirement for it (since the only symbols required
for it are in `libc++experimental.a`).

However, some parts of std::format depend indirectly on the floating
point std::to_chars implementation, which does have deployment target
requirements.

This patch removes all the availability format for std::format and
updates the XFAILs in the tests to properly explain why they fail
on old deployment targets, when they do. It also changes a couple
of tests to avoid depending on floating-point std::to_chars when
it isn't fundamental to the test.

Finally, some tests are marked as XFAIL but I added a comment saying

TODO FMT This test should not require std::to_chars(floating-point)

These tests do not fundamentally depend on floating-point std::to_chars,
however they end up failing because calling std::format even without a
floating-point argument to format will end up requiring floating-point
std::to_chars. I believe this is an implementation artifact that could
be avoided in all cases where we know the format string at compile-time.
In the tests, I added the TODO comment only to the places where we could
do better and actually avoid relying on floating-point std::to_chars
because we know the format string at compile-time.

Differential Revision: https://reviews.llvm.org/D134598

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[libc++][format] Fixes visit_format_arg.

The Standard specifies which types are stored in the basic_format_arg
"variant" and which types are stored as a handle. Libc++ stores
additional types in the

[libc++][format] Fixes visit_format_arg.

The Standard specifies which types are stored in the basic_format_arg
"variant" and which types are stored as a handle. Libc++ stores
additional types in the "variant". During a reflector discussion
@jwakely mention this is user observable; visit_format_arg uses the type
instead of a handle as argument.

This optimization is useful and will probably be used for other small
types in the future. To be conferment the visitor creates a handle and
uses that as argument. There is a second visitor so the formatter can
still directly access the 128-bit integrals.

The test for the visitor and get has been made public too, there is no
reason not too. The 128-bit integral types are required by the Standard,
when they are available.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D138052

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[libc++][format] Add __format_arg_store.

This implements the struct `__format_arg_store` and its dependencies:
* the class basic_format_arg,
* the class basic_format_args,
* the class basic_format_c

[libc++][format] Add __format_arg_store.

This implements the struct `__format_arg_store` and its dependencies:
* the class basic_format_arg,
* the class basic_format_args,
* the class basic_format_context,
* the function make_format_args,
* the function wmake_format_args,
* the function visit_format_arg,
* several Standard required typedefs.

The following parts will be implemented in a later patch:

* the child class `basic_format_arg::handle`,
* the function `basic_format_arg::basic_format_arg(const T* p)`.

The following extension has been implemented:
* the class basic_format_arg supports `__[u]int128_t` on platform where libc++ supports 128 bit integrals.

Implements parts of:
* P0645 Text Formatting

Completes:
* LWG3371 visit_format_arg and make_format_args are not hidden friends
* LWG3542 basic_format_arg mishandles basic_string_view with custom traits

Note https://mordante.github.io/blog/2021/06/05/format.html gives a bit more information about the goals and non-goals of this initial patch series.

Reviewed By: #libc, ldionne, vitaut

Differential Revision: https://reviews.llvm.org/D103357

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