list.append seems to break return type checker

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 !

Reproduce the two snippets with mypy 0.770 on Python 3.8 and compare the inferred type of listings before the return. Trace the handling of list.append versus the list literal, then add a regression test showing that both cases produce the expected Optional[List[int]] return-value error.