list.append seems to break return type checker

[AugPro]

Hello, concerning the function below:

from typing import List
def foo() -> List[int]:
    listings = None
    if 2 != 2:
        listings = list()
        listings.append(1)
    return listings

I expected:
Incompatible return value type (got "Optional[List[int]]", expected "List[int]")

I got no error

This one does throw the expected error (same config):

from typing import List
def foo() -> List[int]:
    listings = None
    if 2 != 2:
        listings = [1]
    return listings

running mypy 0.770 on python 3.8

I would appreciate it if you could tell me if I am doing something wrong :)

Have a good day !

[msullivan]

Yeah that looks like a bug. Something must be wrong with the case where we try to refine a partial type multiple times in a conditional way?

[AugPro]

I've never contributed to this repo but if you want I'd be glad to help!