type narrowing leads to "unreachable" code being actually reachable

Bug Report

To Reproduce

class Foo:
    def foo(self) -> None:
        print("Foo!!!")

class Bar:
    def __init__(self) -> None:
        self.foo: Foo | None = Foo()

    def get_rid_of_foo(self) -> None:
        self.foo = None


bar = Bar()
if bar.foo is not None:
    bar.get_rid_of_foo()
    assert bar.foo is None
    print(1 + "lol")

Expected Behavior

An error for the obviously wrong print(1 + "lol"). It fails there at runtime.

Actual Behavior

no errors!

Your Environment

mypy 0.942 with --python-version 3.10

Start by running the reproducer with mypy 0.942 and --python-version 3.10, then trace how narrowing around bar.foo behaves after get_rid_of_foo(). Done means mypy reports the invalid 1 + "lol" expression while preserving valid narrowing behavior.