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

<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>

Fixes https://github.com/llvm/llvm-project/issues/128676

Implements https://wg21.link/P2835R7

Closes #118377

# References

- https://wg21.link/atomics.ref.generic.general
- https://wg21.link/atomics.ref.int
- https://wg21.link/atomics.ref.float
- https://wg21.link/atomics.ref.pointer

---------

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

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>

Implements [P3044R2](https://wg21.link/P3044R2)

Note: `substr.pass.cpp` is refactored to accommodate the test of
`basic_string_view`'s `subview` which is an alias of `substr` without
changing the test cases.

Closes #148140

# References

- https://github.com/cplusplus/draft/pull/7975
- https://wg21.link/string.substr
- https://wg21.link/string.view.ops

---------

Co-authored-by: Hristo Hristov <zingam@outlook.com>
Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>

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

And constrain the new `operator==` since C++26.

This patch implements parts of P2165R4, P2944R3, and a possibly improved
resolution of LWG3882. Currently, libstdc++ and MSVC STL constrain the
new overloads in the same way.

Also set feature-test macro `__cpp_lib_constrained_equality` and add
related release note, as P2944R3 will completed with this patch.

Fixes #136765
Fixes #136770
Fixes #105424

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