TBH, I don't like this idea. Consider this situation:

    @singledispatch
    def what(x: Iterable) -> None:
        print('general case')

    @what.register
    def _(x: Sequence[int]) -> None:
        print('special case')

    what(['is', 'going', 'on'])  # special case?

Even if you put in the docs that variables are erased etc. people will assume type arguments mean something unless rejected by `singledispatch`. The behaviour you propose can cause confusion.