Folding algorithms

[JMazurkiewicz]

This PR implements P2322R6 (ranges::fold) and closes #2922.

Changes:

• The <algorithm> header includes <optional> in C++23 mode,
• Added folding functions:
  • ranges::fold_left and ranges::fold_left_with_iter - both invoke _Fold_left_with_iter_fn::_Fold_left_with_iter_impl,
  • ranges::fold_left_first and ranges::fold_left_first_with_iter - both invoke _Fold_left_first_with_iter_fn::_Fold_left_first_with_iter_impl,
  • ranges::fold_right - invokes _Fold_right_unchecked
  • ranges::fold_right_last - invokes private function _Fold_right_last_unchecked

[strega-nil-ms]

Minor comments for the algorithm, but otherwise this looks fantastic!

[StephanTLavavej]

Thanks, this looks great! 😻 I exhaustively compared this to the Standard, and checked all of the _EXPORT_STD markings. All I found were some very minor nitpicks and a couple of test typos, so I validated and pushed changes. FYI @strega-nil-ms as I pushed changes after you approved.

[timsong-cpp]

Ideally we'd want this to avoid the extra move into the in_value_result and back out. (This is why the wording uses Returns: instead of "Effects: Equivalent to return ...;")

[StephanTLavavej]

Do RVO or deferred temporary materialization help with that?

I think this would be reasonable to address in a followup PR.

[StephanTLavavej]

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

[StephanTLavavej]

I've pushed a merge with main to resolve trivial adjacent-add conflicts in yvals_core.h and tests/std/test.lst.

[StephanTLavavej]

Thanks for implementing this C++23 feature! ⚙️ 🎉 😻