I think it’s a bug, I’m having the same issue here
parametric_class(t:type) := class<computes>:
SomeMethod : tuple() -> t
ReturnInt():int=0
SomeParametricClassInt : parametric_class(int) = parametric_class(int):
SomeMethod := ReturnInt # Script error 3502: Using a value of type tuple()->int as a value of type tuple()->t is not yet implemented.