[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++] Remove transitive includes from empty headers (#116295)

This removes transitive includes that are only in a header that is empty
in a given C++ version.

[libc++] Include the rest of the detail headers by version in the umbrella headers (#96032)

This is a follow-up to #83740.

[libc++] P2389R2: `dextents` Index Type Parameter (#97393)

This patch implements P2389R2, which was adopted at the St. Louis meeting.
It builds upon previous enhancements from P2299R3, which introd

[libc++] P2389R2: `dextents` Index Type Parameter (#97393)

This patch implements P2389R2, which was adopted at the St. Louis meeting.
It builds upon previous enhancements from P2299R3, which introduced deduction
guides and the `dextents` alias template.

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[libc++] P3029R1: Better `mdspan`'s CTAD (#87873)

## Abstract

This pull request implements [P3029R1](https://wg21.link/P3029R1). The
paper discusses the current behavior of `mdspan`'s most commo

[libc++] P3029R1: Better `mdspan`'s CTAD (#87873)

## Abstract

This pull request implements [P3029R1](https://wg21.link/P3029R1). The
paper discusses the current behavior of `mdspan`'s most common
pointer-indices CTAD, where the `Extents` template parameter is deduced
as `dextents` (dynamic extents), even when passing compile-time constant
values. The author believes this behavior is suboptimal, as it doesn't
take advantage of the compile-time information. The proposed change
suggests deducing static extents if `integral_constant`-like constants
are passed, resulting in more intuitive syntax and less error-prone
code.

## Reference

- [P3029R1](https://wg21.link/P3029R1)
- [Draft C++ Standard: [span.syn]](https://eel.is/c++draft/span.syn)
- [Draft C++ Standard: [mdspan.syn]](https://eel.is/c++draft/mdspan.syn)

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[libc++] LWG-4021 "`mdspan::is_always_meow()` should be `noexcept`", use `LIBCPP_STATIC_ASSERT` for `noexcept` strengthening (#74254)

Found while running libc++'s test suite with MSVC's STL.

* I'

[libc++] LWG-4021 "`mdspan::is_always_meow()` should be `noexcept`", use `LIBCPP_STATIC_ASSERT` for `noexcept` strengthening (#74254)

Found while running libc++'s test suite with MSVC's STL.

* I've filed [LWG-4021](https://cplusplus.github.io/LWG/issue4021)
"`mdspan::is_always_meow()` should be `noexcept`" and implemented this
in libc++'s product and test code.
* Use `LIBCPP_STATIC_ASSERT` to avoid issues with `noexcept`
strengthening in MSVC's STL.
+ As permitted by the Standard, MSVC's STL conditionally strengthens
`mdspan` construction/`is_meow`/`stride` and `elements_view` iterator
`base() &&`, and always strengthens `basic_stringbuf` `swap`.
+ In `mdspan/properties.pass.cpp`, this also upgrades runtime `assert`s
to `static_assert`s.
* Improvement: Upgrade `assert` to `static_assert` when inspecting the
`noexcept`ness of `std::ranges::iter_move`. (These `!noexcept` tests
weren't causing issues for MSVC's STL, so I didn't change them to be
libc++-specific.)

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[libc++] Updates mdspan synopsis,

The class layout_stride has been implemented in
639a0986f3a34d52533a726e2a4193411cc1312f

[libc++] mdspan - implement layout_stride (#69650)

This implements layout_stride for C++23 and with that completes the
implementation of the C++23 mdspan header. The feature test macro is
added, a

[libc++] mdspan - implement layout_stride (#69650)

This implements layout_stride for C++23 and with that completes the
implementation of the C++23 mdspan header. The feature test macro is
added, and the status pages updated.

Co-authored-by: Damien L-G <dalg24@gmail.com>

Differential Revision: https://reviews.llvm.org/D157171

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[libc++][nfc] Fixes emacs magic marker.

[libc++][mdspan] Implement std::mdspan class

This implements P0009 std::mdspan ((https://wg21.link/p0009)),
a multidimensional span with customization points for
layouts and data access.

Co-authore

[libc++][mdspan] Implement std::mdspan class

This implements P0009 std::mdspan ((https://wg21.link/p0009)),
a multidimensional span with customization points for
layouts and data access.

Co-authored-by: Damien L-G <dalg24@gmail.com>

Differential Revision: https://reviews.llvm.org/154367

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[libc++][mdspan] Implement default_accessor

This commit implements default_accessor in support of C++23 mdspan
(https://wg21.link/p0009). default_accessor is the trivial accessor
using plain pointer

[libc++][mdspan] Implement default_accessor

This commit implements default_accessor in support of C++23 mdspan
(https://wg21.link/p0009). default_accessor is the trivial accessor
using plain pointers and reference to element types.

Co-authored-by: Damien L-G <dalg24@gmail.com>

Differential Revision: https://reviews.llvm.org/D153935

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[libc++][mdspan] Implement layout_left

This commit implements layout_left in support of C++23 mdspan
(https://wg21.link/p0009). layout_left is a layout mapping policy
whose index mapping corresponds

[libc++][mdspan] Implement layout_left

This commit implements layout_left in support of C++23 mdspan
(https://wg21.link/p0009). layout_left is a layout mapping policy
whose index mapping corresponds to the memory layout of Fortran arrays.
Thus the left most index has stride-1 access, and the right most index
is associated with the largest stride.

Co-authored-by: Damien L-G <dalg24@gmail.com>

Differential Revision: https://reviews.llvm.org/D153783

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[libc++][mdspan] Implement layout_right

This commit implements layout_right in support of C++23 mdspan
(https://wg21.link/p0009). layout_right is a layout mapping policy
whose index mapping correspo

[libc++][mdspan] Implement layout_right

This commit implements layout_right in support of C++23 mdspan
(https://wg21.link/p0009). layout_right is a layout mapping policy
whose index mapping corresponds to the memory layout of multidimensional
C-arrays, and is thus also referred to as the C-layout.

Co-authored-by: Damien L-G <dalg24@gmail.com>

Differential Revision: https://reviews.llvm.org/D151267

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[libcxx] Add mdspan/extents

This patch adds std::extents. extents is one of the core classes used by std::mdspan. It describes a multi-dimensional index space with a mix of compile time and runtime

[libcxx] Add mdspan/extents

This patch adds std::extents. extents is one of the core classes used by std::mdspan. It describes a multi-dimensional index space with a mix of compile time and runtime sizes. Furthermore, it is templated on the index type used to describe the multi-dimensional index space.

The class is designed to be highly optimizable in performance critical code sections, and is fully useable in constant expressions contexts.

Testing of this class tends to be somewhat combinatorical, due to the large number of possible corner cases involved in situations where we have both runtime and compile time extents. To add to this, the class is designed to be interoperable (in particular constructible) from arguments which only need to be convertible to the index_type, but are otherwise arbitrary user types. For a larger discussion on the design of this class refer to: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0009r18.html

Co-authored-by: Damien L-G <dalg24@gmail.com>

Reviewed By: ldionne, #libc

Spies: libcxx-commits, H-G-Hristov, tschuett, philnik, arichardson, Mordante, crtrott

Differential Revision: https://reviews.llvm.org/D148067

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