Redefinition from None not an error -- a bug?

[iutlu]

The following code

def test(param: Optional[int]) -> List[str]:
    ret = None  # Revealed type is 'None'
    if param is not None:
        ret = []
        ret.append('')  # Revealed type is 'builtins.list[builtins.str]'
    return ret

does not raise any errors, which looks like a bug since the return value is not necessarily a List[str] -- tested with mypy 0.750, Python 3.7.

mypy.ini:

no_implicit_optional = True

[ilevkivskyi]

I think this is a duplicate of #4088 (which itself btw may be also related to #5423).