P0655R1 visit<R>()

[NathanSWard]

Description

This is an implementation of std::visit<R>.
Addresses issue #31

Checklist

• I understand README.md. I also understand that acceptance of
  community PRs will be delayed until the test and CI systems are online.
• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before CI is online, leave this unchecked for
  initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
  I changed the underlying machinery of std::variant to not assume the return type to be _Variant_visit_result_t but rather have it be a templated parameter that is passed down by the calling std::visit function. I don't believe this causes ABI breaks, but am unsure.
• These changes were written from scratch using only this repository and
  the C++ Working Draft as a reference (and any other cited standards).
  If they were derived from a project that's already listed in NOTICE.txt,
  that's fine, but please mention it. If they were derived from any other
  project (including Boost and libc++, which are not yet listed in
  NOTICE.txt), you must mention it here, so we can determine whether the
  license is compatible and what else needs to be done.

[NathanSWard]

This is the example test case I used for this implementation. It directly mimics WG21-P0655's example cases.

#include <variant>
#include <type_traits>

template <class I>
using O = I;

struct process {
    template<class I>
    constexpr auto operator()(I i) -> O<I> {
        return {};
    }
};

int main() {
    using I1 = int;
    using I2 = double;

    std::variant<I1, I2> input = {};

    auto output = std::visit<std::variant<O<I1>, O<I2>>>(process{}, input);
    auto result = std::visit<std::common_type_t<O<I1>, O<I2>>>(process{}, input);
    std::visit<void>(process{}, input);
}

[NathanSWard]

"fixup"-ed typo commit to original commit to make the entire commit easier to read.

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephanTLavavej]

I have only minor stylistic comments remaining. The rest looks good to me (although I am not a variant expert). Thanks!

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[CaseyCarter]

This looks good to me as well. I'll throw some test coverage together.

[CaseyCarter]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[BillyONeal]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[BillyONeal]

LGTM pending the usual test caveats in this repo at the moment.

[StephanTLavavej]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[CaseyCarter]

All green to merge. Thanks, @NathanSWard - great work!

[BillyONeal]

Thanks!