How do I pass the file's type as a function parameter?

Question

I am trying to write a custom copy-data function. I would like to give it a stronger typing so I can get the auto-complete. In other words, I have this:

# res://MyClass.gd
func copyFrom( other ):
    value = other.value
    # ...

but would like this:

# res://MyClass.gd
func copyFrom( other : MyClass ):
    value = other.value
    # ...

I have tried a few things with these frustrating findings:
It looks like you can't preload a class in it's own file.
It looks like load results can't be made const, so they can't be used as a type, (I think?)
I would add class_name but that seems to give me all kinds of circular errors if I try to use the file anywhere else.

There may not be a way to do it right now (3.1), and that's fine, too. I just would like to know the answer if it is possible.