Exclude match aliases from partial unification attempts

[odersky]

Fixes #15983

[smarter]

I don't understand what's specific about aliases to match types here, surely we should still be able to construct type lambdas involving them?

[odersky]

Well, no, it crashes in the partial unification code otherwise.

[odersky]

I think we require that a match type alias is always fully applied.

[smarter]

I think we require that a match type alias is always fully applied.

This doesn't seem to be required in sources, using the same TupleUnionLub match type from the test case I can write:

class Foo[F[_]]
val x = new Foo[[X] =>> TupleUnionLub[Tuple, Any, X]]

I would expect partial unification to generate type lambdas like this too.

[odersky]

OK, so do you want to take a look at it?

[smarter]

I'll assign myself to the issue.

[odersky]

Issue has been open for a month now.

[smarter]

I did not have a job during that month.