When checking the following code:

from typing import AbstractSet, Iterable, Optional, Set, TypeVar

T = TypeVar('T')

def intersection(seq: Iterable[AbstractSet[T]]) -> Optional[Set[T]]:
    ret = None
    for elem in seq:
        if ret is None:
            ret = set(elem)
        else:
            ret &= elem
    return ret

mypy reports:

$ mypy --warn-unreachable intersection.py
intersection.py:11: error: Statement is unreachable

This statement is the ret &= elem inside the else block.

It seems that mypy is using the initial type of ret to conclude that ret is None is always true, ignoring the fact that inside the loop ret is redefined to a different type.

If I place a reveal_type just before the return, mypy does report Union[builtins.set[T`-1], None] as the type, so it figures out the redefinition eventually.

I'm using mypy 0.770 on Pyton 3.8.2, with the --warn-unreachable flag.