False positive: when using optional callable function in lambda after asserting callable is not None

[HiroIshida]

Bug Report
When function takes optional callable, even though callable is asserted that is is not None, mypy says error: "None" not callable when used in a lambda function.

To Reproduce

from typing import Callable, Optional, List

def example(fun: Optional[Callable[[int], float]]) -> None:
    assert fun is not None
    sorted([1, 2], key=lambda x: fun(x))

Expected Behavior
no warning

Actual Behavior
show the following warning

h-ishida@stonep:~/tmp$ mypy mypy_error.py 
mypy_error.py:6: error: "None" not callable  [misc]

Your Environment

• Mypy version used: 0.990+dev.9783b46a0062b6acfb8ba58e349e811873fc13f2
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.10

[hauntsaninja]

https://mypy.readthedocs.io/en/stable/common_issues.html#narrowing-and-inner-functions / see #2608