[Bug c++/124457] spurious reflection cannot be applied to a placeholder type error in constrained return types

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124457

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
Part of the issue with AlsoTrue is that we still represent the
return-type-requirement as a constrained auto, as a legacy of the Concepts TS
implementation, but that's mostly a distraction...

...because the f3 case is in fact a placeholder, and I still think it should be
accepted; we shouldn't reject a type-constraint based on where it's used. 
We'll eventually take the reflection of the deduced type.

make_constrained_placeholder_type could do something to obscure the
placeholderness across the call to build_concept_check?