P2278R4: cbegin should always return a constant iterator ("Iterators" section only)

[JMazurkiewicz]

Implements "Iterators" section from P2278R4 - cbegin should always return a constant iterator. Towards #2918.

[hewillk]

Just curious, does the current implementation pass the following two assertions?

using CI = std::basic_const_iterator<int*>;
static_assert(std::sentinel_for<CI, int*>);
static_assert(std::sized_sentinel_for<CI, CI>);

[JMazurkiewicz]

Just curious, does the current implementation pass the following two assertions?

Nope, it doesn't.

There was short discussion on discord about it: https://discord.com/channels/737189251069771789/738835809238646854/1011456843106762892.

[CaseyCarter]

Heads up: I pushed some commits that (1) implement the proposed resolution of LWG-3769, which is intended to correct the constraint recursion in the design, and (2) workaround LLVM-55945 to avoid constraint recursion in practice.

[CaseyCarter]

@JMazurkiewicz is this PR ready to become non-draft now?

[JMazurkiewicz]

@CaseyCarter yes, it is. And thank you for implementing LWG-3769!

[frederick-vs-ja]

LWG-3765 has been set to Tentatively Ready, so I think it's safe to implement the resolution without comments.

Feel free to do this in a separated PR. I'm not sure whether the constraints should be tested.

[StephanTLavavej]

Thanks for adding a visualizer! 😸

Mirroring note: As usual, this will need to be triple-mirrored to the VS repo, but I'm ok with deferring that until the next time we have a decent-sized batch of visualizer updates.

[StephanTLavavej]

Thanks for implementing this part of the feature! 😻 FYI @strega-nil-ms, I pushed changes after you approved.

I have a major outstanding question about HasPeek in the test, where I'm 80% sure we're silently missing test coverage, and a minor question about the comment change in range_algorithm_support.hpp. No remaining concerns with the product code.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for constantly making the STL better! 😹 😻 😸