Elision Only types can't be reflected in MSVC > 16.28

[chjanke]

MSVC implements mandatory copy elision in v16.29.

I have expected that copy elision also enables reflecting aggregates that are neither copyable nor movable.
Clang and GCC both successfully reflect such types, however MSVC fails to do so.

Minimal example to reproduce the issue: godbolt

As shown in the example, the aggregate can be returned from functions just fine through mandatory copy elision, and it is also classified as an aggregate and can be aggregate initialized as expected.
However MSVC fails, asking me to report the issue, so here it is.

[chjanke]

This seems to be a compiler bug in msvc where the C++17 elision rules don't correctly work inside decltype.
Related report: https://developercommunity.visualstudio.com/t/C17-Guaranteed-Copy-Elision-does-not-a/10374637

A workaround for this is to declare the move constructor of your type private and not implement it.
The existence of the signature, or rather it not being explicitly deleted, is enough to make Boost.PFR work again.