constexpr INVOKE and related utilities

[AdamBucior]

Resolves #51.

BTW without the checklist PRs feel so empty.

[StephanTLavavej]

Thanks for the feature! I've exhaustively reviewed the product code but haven't really looked at the test coverage yet; I'll take a look at the tests during the next iteration.

[AdamBucior]

BTW shouldn't _Invoker_ret and _Unforced be moved to <functional> if they're used only by std::function and bind()?

[AdamBucior]

I don't understand why some of the x86 tests are failing.

[cbezault]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[AdamBucior]

I think it's ready for initial review

[StephanTLavavej]

This looks good! My apologies for taking a month to get back to this. I found a couple of quiet hours to think hard about the changes, and everything appears to be in order. (Not easy to achieve, given the complexity of bind's machinery - I really appreciate your work here.) I'll push changes for some pre-existing/superficial syntactic things I noticed, and a bit more testing for conditionally noexcept bind(), and then I think this will be ready for final review and merge (to ship in VS 2019 16.8 Preview 1 if we can merge this week, otherwise Preview 2).

[StephanTLavavej]

Mini-changelog for more tests for conditionally noexcept bind():

• Unwrapping a reference_wrapper is noexcept, but passing the unwrapped reference to the callable object might involve a possibly throwing conversion.
• Forwarding through a placeholder is noexcept, but passing the unbound argument to the callable object might involve a possibly throwing conversion.
• Nested bind() needs to sense whether the nested call might throw.

Mark PossiblyThrowingInt(int) as constexpr so we can construct possibly_throwing_int during constant evaluation; ref(possibly_throwing_int) is simpler than declval<reference_wrapper<PossiblyThrowingInt>>().

[StephanTLavavej]

There seems to be a totally unrelated infrastructure issue with the arm/arm64 CI builds that we'll need to investigate, but I believe this PR is crystalline perfection. If a second reviewer can be equally convinced, I'll try to port this for Preview 1. 😺

[CaseyCarter]

One style comment, but I'm happy to merge this either way.

[StephanTLavavej]

Thanks again for implementing this C++20 feature (over 3% of the remaining features, by paper count!). This will ship in VS 2019 16.8 Preview 1. 😺 😸 🎆