Wildcards in type synonyms

[ÉRDI Gergő]

Hi,

I'd like to implement type synonyms containing wildcards. The idea is
that if you have `type MySyn a = MyType a _ Int`, then during
typechecking, every occurrence of `MySyn T` would be expanded into
`MyType T w123 Int`, with a fresh type (meta)variable `w123`.

One worrying thing I noticed in my initial exploration of the GHC
codebase is that the Core representation of `Type`s can still contain
type synonym occurrences. And this doesn't seem like just an artefact
of sharing the `Type` representation with `TcType`, since the
`coreView` function also has code to look through type synonyms.

What is the reason for this? I would have expected type synonyms to be
only relevant during typechecking, and then fully resolved in the
elaborated Core output. If that were the case, then a new version of
`expand_syn` could live in `TcM` and take care of making these fresh
metavariables.

Beside this concrete question, taking a step back, I would also like
to hear from people who know their way around this part of GHC, what
they think about this and how they'd approach implementing it.

Thanks,
         Gergo