P2321R2: Added std::views::zip_transform.

[tylerbrawl]

As a follow up to #3035, this PR implements std::views::zip_transform from P2321R2 as described in the C++ Working Draft. The following defect reports (DRs) and revisions from #2252 have also been implemented:

• P2494R2 (This handles std::views::zip_transform only, since Implement P2494R2 Relaxing Range Adaptors To Allow Move-Only Types #2965 already implemented the paper for existing ranges in the MSVC STL.)
• LWG-3702
• LWG-3773

One thing to note about the current implementation is that in [range.zip.transform.overview]/2.1.2 of the C++ Working Draft, it is written that ((void)F, auto(views::empty<decay_t<invoke_result_t<FD&>>>)) is a potential equivalent expression when using the std::views::zip_transform customization point object. This makes use of the decay-copy language feature proposed in P0849R8, but since that hasn't been implemented yet in the MSVC, the current implementation instead gets the relevant type manually. I would like further input on approaching this problem (use the current solution, wait for compiler support, use the older decay-copy() function, etc.).

Lastly, this will be marked as a draft release until test cases are implemented. As always, let me know if you have any questions.

[frederick-vs-ja]

I have some thoughts on empty_view and iterator_category...

[frederick-vs-ja]

Some changes are needed to conform to [range.zip.transform.overview]/2.

[strega-nil-ms]

Minor changes requested

[StephanTLavavej]

Pushed changes to address @strega-nil-ms's comments, plus minor things I noticed along the way:

• Rename to _Subscript_closure.
• Rename to _Get_iterator_inner.
  • Also drop a couple of unnecessary explicit template arguments.
• Shorten _Iter_sent_difference_type (to avoid wrapping weirdly).
• Remove clang-format suppressions.
• Use C-style comments for /* strengthened */ to avoid wrapping braces on the next line.
• Change _Is_end_noexcept to be a _CONSTEVAL function.

[StephanTLavavej]

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

[StephanTLavavej]

I had to push an additional commit to work around an internal compiler assertion, VSO-1732554 "x86chk assertion failure in immediately invoked lambda computing iterator_category: RPF_ISSET(TypeSeenThisDepth) || Parser::Utility::CurrentlyInRdParser(), p1inl.c 5344". The workaround is to use a chain of conditional_ts.

[StephanTLavavej]

I had to push another commit to work around #1030. The internal build uses native compilers for x86 (eww 🙀 🤮) and /analyze's higher memory consumption exceeds 4 GB for at least the random-access configuration (and is dangerously high for the others).

The really expensive thing is the matrix of 8 permutations. Instead of skipping everything for /analyze, I chose to skip 7 out of 8, to leave at least some coverage. (I'm not too worried about /analyze-specific warnings or problems, given this residual coverage.)

[CaseyCarter]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

Thanks for implementing another major view in this paper! 🤐 🔀 😻