Actually, on further consideration, you could restrict the ' to only occur where the type parameter is introduced, like so: List<X> foo(List<'X> inp) = .

Actually, on further consideration, you could restrict the ' to only occur where the type parameter is introduced, like so: List<X> foo(List<'X> inp) = ... List<X> bar(List<'X> xs) = ... # new X scoped to this declaration List<X> baz(List<X> xs) = ... # use foo's X ...