Resolves #148131

- Unlock `std::optional<T&>` implementation
- Allow instantiations of `optional<T(&)(...)>` and `optional<T(&)[]>`
but disables `value_or()` and `optional::iterator` + all `iterator`
related functions
- Update documentation
- Update tests

Implemented
[[*time.traits.is.clock*]](https://eel.is/c++draft/time.traits.is.clock)
from [P0355R7](https://wg21.link/p0355r7).

This patch implements the C++20 feature `is_clock` and `is_clock_v`
based on the documentation [on
cppreference](https://en.cppreference.com/w/cpp/chrono/is_clock.html)

Fixes #166049.

This commit adds move constructor, move assignment and `swap`
to `exception_ptr`. Adding those operators allows us to avoid
unnecessary calls to `__cxa_{inc,dec}rement_refcount`.

Performance results (from libc++'s CI):

```
Benchmark                               Baseline    Candidate    Difference    % Difference
------------------------------------  ----------  -----------  ------------  --------------
bm_exception_ptr_copy_assign_nonnull        9.77         9.94          0.18           1.79%
bm_exception_ptr_copy_assign_null          10.29        10.65          0.35           3.42%
bm_exception_ptr_copy_ctor_nonnull          7.02         7.01         -0.01          -0.13%
bm_exception_ptr_copy_ctor_null            10.54        10.60          0.06           0.56%
bm_exception_ptr_move_assign_nonnull       16.92        13.76         -3.16         -18.70%
bm_exception_ptr_move_assign_null          10.61        10.76          0.14           1.36%
bm_exception_ptr_move_ctor_nonnull         13.31        10.25         -3.06         -23.02%
bm_exception_ptr_move_ctor_null            10.28         7.30         -2.98         -28.95%
bm_exception_ptr_swap_nonnull              19.22         0.63        -18.59         -96.74%
bm_exception_ptr_swap_null                 20.02         7.79        -12.23         -61.07%
```

As expected, the `bm_exception_ptr_copy_*` benchmarks are not influenced by
this change. `bm_exception_ptr_move_*` benefits between 18% and 30%. The
`bm_exception_ptr_swap_*` tests show the biggest improvements since multiple
calls to the copy constructor are replaced by a simple pointer swap.

While `bm_exception_ptr_move_assign_null` did not show a regression in the CI
measurements, local measurements showed a regression from 3.98 to 4.71, i.e. by
18%. This is due to the additional `__tmp` inside `operator=`. The destructor
of `__other` is a no-op after the move because `__other.__ptr` will be a
nullptr. However, the compiler does not realize this, since the destructor is
not inlined and is lacking a fast-path. As such, the swap-based implementation
leads to an additional destructor call. `bm_exception_ptr_move_assign_nonnull`
still benefits because the swap-based move constructor avoids unnecessary
__cxa_{in,de}crement_refcount calls. As soon as we inline the destructor, this
regression should disappear again.

Works towards #44892

<https://wg21.link/P2641R4>

Implements the C++26 function in `<type_traits>` [meta.const.eval] (and
the corresponding feature test macro `__cpp_lib_is_within_lifetime`)

```c++
template<class T>
  consteval bool is_within_lifetime(const T*) noexcept;
```

This is done with the `__builtin_is_within_lifetime` builtin added to
Clang 20 by #91895 / 2a07509c8d3c8b5b2c88e4f73dde0071bf506870. This is
not (currently) available with GCC.

This implementation has provisions for LWG4138
<https://cplusplus.github.io/LWG/issue4138> where it is ill-formed to
instantiate `is_within_lifetime<T>` with a function type `T`.

Closes #105381

Co-authored-by: Mital Ashok <mital@mitalashok.co.uk>

Implements [P3060R3](https://wg21.link/P3060R3)

Closes #148175

# References

- https://github.com/cplusplus/draft/issues/7966
- https://github.com/cplusplus/draft/pull/8006
- https://wg21.link/customization.point.object
- https://wg21.link/range.iota.overview
- https://wg21.link/ranges.syn

---------

Co-authored-by: Hristo Hristov <zingam@outlook.com>
Co-authored-by: A. Jiang <de34@live.cn>

Resolves #105430

- Implement all required pieces of P3168R2
- Leverage existing `wrap_iter` and `bounded_iter` classes to implement
the `optional` regular and hardened iterator type, respectively
- Update documentation to match

Fixes #104977
Fixes #105035

---------

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
Co-authored-by: A. Jiang <de34@live.cn>

Fixes #105260

This patch applies the change as a DR to C++20. The rationale is that
the paper is more like a bug fix. It does not introduce new features, it
simply changes an existing behaviour (as a bug fix). MSVC STL DRed this
paper to C++20 as well.

* Implement "P2441R2 `views::join_with`" (https://wg21.link/P2441R2),
closes #105185
* Implement LWG4074 (https://wg21.link/LWG4074), closes #105346
* Complete implementation of "P2711R1 Making multi-param constructors of
views explicit" (https://wg21.link/P2711R1), closes #105252
* Complete implementation of "P2770R0 Stashing stashing iterators for
proper flattening" (https://wg21.link/P2770R0), closes #105250

Closes #118372