Support For Incomplete Types In reference_wrapper

[AdamBucior]

Description

Resolves #14.

Checklist

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[CaseyCarter]

This looks fine to me; I'm sure we'll have some test fallout to fix in addition to adding some incomplete type coverage.

[BillyONeal]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[BillyONeal]

This change is blocked by the libcxx test utilities\function.objects\refwrap\weak_result.pass.cpp which fails after the typedefs are removed. I think we need to submit a change adding // REQUIRES: c++98 || c++03 || c++11 || c++14 || c++17 to that test or similar, I'm figuring out how to do that.

[BillyONeal]

Well that was fast. Thanks @tstellar ! llvm/llvm-project@85ee4ff

[StephanTLavavej]

Thank you, this looks good!

[StephanTLavavej]

There's a design question here, because we haven't yet merged #380 to implement #28 (WG21-P0619 Removing C++17-Deprecated Features In C++20). Specifically, an escape hatch to restore reference_wrapper's weak result type is incompatible with supporting incomplete types. There are two possible decisions: allow reference_wrapper to be affected by such an escape hatch (thereby disabling support for incomplete types), or exempt reference_wrapper from the escape hatch (so C++20 mode always activates support for incomplete types).

It's not a big deal because reference_wrapper's support for weak result types is extremely obscure. I am inclined to accept the code as-is, meaning that C++20 mode always makes reference_wrapper support incomplete types and removes its weak result type, even if weak result types have otherwise been escape-hatched.

[BillyONeal]

Thanks for your contribution!