LWG-2774 std::function construction vs assignment

[sam20908]

Partially addresses #1965

[fsb4000]

Compile-only tests should be test.compile.pass.cpp

[fsb4000]

You should also edit the file: https://github.com/Microsoft/STL/blob/main/tests/std/test.lst
(add tests\LWG2774_function_construction_vs_assignment)

[sam20908]

It appears that the following somehow became true when it's supposed to be false:

static_assert(is_constructible_v<decay_t<_Fx>, _Fx>, "decay_t<F> shall be constructible with F");

However, if I pass in a Fn instance instead (I added Fn() = default), then the assertion will pick it up:

void test() {
    const Fn fn;
    function<void()> f{fn}; // fail to compile here because of the assertion
}

I am unsure how this can be...

EDIT: Oh, maybe since the compiler technically found a candidate (not at the assertion yet), is_constructible_v becomes true?

[fsb4000]

yes, you are correct:
https://gcc.godbolt.org/z/zjqvTc38W

[sam20908]

I am making this a draft right now as there seems to be no way of testing a valid instantiation with failed assertion (the detector pattern implemented is currently malfunctional too).

[fsb4000]

Looks good. It looks like we really can't create tests for that.

[StephanTLavavej]

Thanks - and sorry again for taking so long to get to this 😿. Your changes are completely correct as far as I can tell 😻, so to move this forward to Final Review I've pushed a merge with main (no conflicts, but there have been a lot of commits in main including submodule changes so it was time), plus changes for a couple of minor codebase convention issues, and a SFINAE cleanup to match the new Standardese.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for improving std::function, one of the STL's most popular types! 🚀 🎉 😸