mypy thinks variable is None even if assigned non-None value inside loop

[Akuli]

Bug Report

To Reproduce

def foo() -> None:
 user = None

 while True:
 if user is None:
 user = 123
 else:
 print(1 + "lol")

foo()

Expected Behavior

mypy error + runtime error

Actual Behavior

no mypy error

Your Environment

• Mypy version used: 0.930

[Akuli]

Workaround: user: int | None = None or user: Optional[int] = None

[zhangck981]

I believe the problem might be something around the keywordis. Here are my test cases:
The first one is the case you provided:

And the second test case is:

I changed the keyword is to the operator == and it shows that mypy detects the error. Therefore, I believe the problem can be located with the keyword is.
I will continue working on this by further inspecting the source code and updating my process! However, I am new to mypy and not familiar with that. It would be best if you can provide me with some information that which files I should pay more attention to.

[A5rocks]

Fixed by #18433.