Implement P2328R1 join_view Should Join All Views Of Ranges

[miscco]

Fixes #1985

I am still baffled why array<string, 5>{{{}, "Hello ", {}, "World!", {}}} | views::join was not fixed 😿

[miscco]

Derp, I did not update the can_test boolean that enables the test...

[timsong-cpp]

I am still baffled why array<string, 5>{{{}, "Hello ", {}, "World!", {}}} | views::join was not fixed 😿

I see nothing to fix, other than perhaps making array a view?

I also note that the paper title is incorrectly transcribed.

[CaseyCarter]

I also note that the paper title is incorrectly transcribed.

I'm tempted to veto the Stephan-casing of the title; "Views of Ranges" doesn't have the same meaning as the intended "views of ranges". @StephanTLavavej, could you live with "P2328R11 join_view Should Join All views Of ranges"?

[CaseyCarter]

@miscco FYI I pushed a change that removed all of the _Dummy members from the optional-alike types in <ranges>. We don't need them in C++20, since we can get away with initializing no union member even in a constexpr constructor.

[StephanTLavavej]

Looks good to me, after @timsong-cpp's comments are addressed.

[miscco]

@timsong-cpp I think I managed to get it working as you suggested.

[StephanTLavavej]

Looks good, I'll push a trivial change to take (int) instead of (const int).

[StephanTLavavej]

FYI @CaseyCarter, @miscco and I pushed changes after you approved.

[StephanTLavavej]

Mirrored to internal MSVC-PR-343332. Please notify me if any further changes are pushed to this branch.

[StephanTLavavej]

@miscco @CaseyCarter This failed in the internal test harness so I had to drop the || defined(MSVC_INTERNAL_TESTING) logic.

[StephanTLavavej]

Thanks for joining this project! 😹 🎉 🚀