[dcreager]

I think that's a genuine error, since as you say, `None` is a possible value for `b` according to your signature.

To handle this you would need to use "narrowing" to separately handle the case where `b` is `None`, and the case where it is not:

  def square(a: Union[int, float], b: Optional[int] = 2) -> float:
      if b is None:
          return 0
      else:
          c = a**b
          return c

https://play.ty.dev/97fe4a09-d988-4cc3-9937-8822e292f8d1

(This is not specific to ty, that narrowing check should work in most Python type checkers.)