LWG-3823 Unnecessary precondition for `is_aggregate`

[StephanTLavavej]

LWG-3823 Unnecessary precondition for is_aggregate

It appears that we'll need to request an MSVC compiler change for this. I haven't checked Clang and EDG's behavior.

[frederick-vs-ja]

I've submitted DevCom-10201896 and LLVM-59002 for this. EDG's __is_aggregate also rejects arrays of incomplete types.

I believe a workaround using disjunction_v is implementable (Godbolt link). Should we temporarily use such a workaround?

[CaseyCarter]

I believe a workaround using disjunction_v is implementable (Godbolt link). Should we temporarily use such a workaround?

As evidenced by my approval of your PR, I think the workaround is fine.