Invalid type error for deferred function

[JukkaL]

Mypy complains about the following program because of deferred type checking:

from typing import Union

def f(a: Union[int, str]) -> None:
    x
    a = x
    a + 1   # Error

x = 1 + 1

If x = 1 + 1 is before the function definition there is no error.

This was originally reported by @ecprice: #1748 (comment). There's additional discussion in the linked PR.

[ilevkivskyi]

This works correctly on master.