[libc++] Allow `vector<bool>` in C++20 ranges

[philnik777]

We currently don't backport overloads to allow vector<bool> to be used in C++20 ranges algorithms. We should consider backporting the corresponding functions to C++20 to allow that. (https://godbolt.org/z/4djbEEfax)

[ldionne]

What paper added those? Was it a LWG issue?

[hewillk]

Hum, why make it work in C++20 given that zip_view and related proxy iterator enhancements come from C++23?

[frederick-vs-ja]

What paper added those? Was it a LWG issue?

It was P2321R2 that made the iterator type of vector<bool> (__bit_iterator in libc++) indirectly_writable. This looks like a pure change in C++23.

[hewillk]

What paper added those? Was it a LWG issue?

It was P2321R2 that made the iterator type of vector<bool> (__bit_iterator in libc++) indirectly_writable. This looks like a pure change in C++23.

libc++ already implemented that, isn’t?

[philnik777]

Hum, why make it work in C++20 given that zip_view and related proxy iterator enhancements come from C++23?

Because this doesn't just interact with zip_view and friends. Currently, vector<bool> can't be used with any writing algorithms, as demonstrated in the godbolt link.

What paper added those? Was it a LWG issue?

It was P2321R2 that made the iterator type of vector<bool> (__bit_iterator in libc++) indirectly_writable. This looks like a pure change in C++23.

libc++ already implemented that, isn’t?

Yes, in C++23. C++20 is broken.

What paper added those? Was it a LWG issue?

I'm not aware of any LWG issue for this.

[frederick-vs-ja]

In microsoft/STL#5848 (comment), @StephanTLavavej and @vmichal seemed inclined to backport the new operator= to all old modes (due to P3612R1 and LWG4187).

(LWG4187 is somehow related but it seemingly only changes bitset::reference.)