Fix #4774: Skip TypeVars and skip if level is equal

[nicolasstucki]

If we have ~t for t: Type[T] used at the same level it is an alias for T.
We disallow it if it is explicit because of PCP but if it is infered it is fine.

[smarter]

We disallow it if it is explicit because of PCP but if it is infered it is fine.

I don't understand the context here but this seems very weird to me. If a type can be inferred it should also be something that the user can write by hand, this has always been the case in Scala so far and is much more user-friendly than the alternative.

[nicolasstucki]

Oh, in this context it can be written by hand. The user can write loop(...) or loop[T](...). In the example, the former gets inferred to loop[~t](...) where ~t is an alias of T, which is fine, we can assume it is just a T. We disallow the ~t as it would not be syntactically PCP.

[Blaisorblade]

@nicolasstucki As I mentioned earlier, I don't believe the PCP actually works until you can explain how it should apply to types without needing all these exceptions.

[nicolasstucki]

This is just an implementation detail that I missed.

[smarter]

~t is an alias of T, which is fine, we can assume it is just a T

I see, that seems OK, but it might be problematic if the ~t leaks into error messages shown to the user

[smarter]

It could also leak when decompiling from Tasty, or in some other ways (e.g. if we add a mode to the IDE to desugar/type-infer an expression).

[nicolasstucki]

Good point. Then the type should be dealiased and replaced.