The type system itself is unsound. For example, this code passes `mypy --strict`, but prints `<class 'list'>` even though `bar` is annotated to return an `int`:
i : int | list[int] = 0
def foo() -> None:
global i
i = []
def bar() -> int:
if isinstance(i, int):
foo()
return i
return 0
print(type(bar()))
