Merge #5124

5124: (Partially) fix handling of type params depending on type params r=matklad a=flodiebold

If the first type parameter gets inferred, that's still not handled correctly; it'll require some more refactoring: E.g. if we have `Thing<T, F=fn() -> T>` and then instantiate `Thing<_>`, that gets turned into `Thing<_, fn() -> _>` before the `_` is instantiated into a type variable -- so afterwards, we have two type variables without any connection to each other.

Co-authored-by: Florian Diebold <flodiebold@gmail.com>