assert not flag -> Cannot determine type of "x" [has-type]

[ikamensh]

Bug Report

Usecase: I found a bug and want to temporarily disable an execution branch in my code. I use assert not flag when I initialize my class. I expect this not to create new type errors.

To Reproduce

https://gist.github.com/mypy-play/c1ba8168e409b445cf187af04a4b42d8

or

class Bar:
    def __init__(self, b: bool) -> None:
        assert not b

        if b:
            self.x = "1"
        else:
            self.x = "2"  # main.py:8: error: Cannot determine type of "x" [has-type]

Expected Behavior

There is no type error in this code.

Actual Behavior

Failed (exit code: 1) (3473 ms)
main.py:8: error: Cannot determine type of "x" [has-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

mypy playground: mypy 1.15.0, python3.12

[Protim44]

Failed (exit code: 1) (3473 ms)
main.py:8: error: Cannot determine type of "x" [has-type]
Found 1 error in 1 file (checked 1 source file)

[sterliakov]

Minimal repro (the class is necessary, doesn't rep with a local variable):

class Bar:
    def __init__(self) -> None:
        if False:
            self.x = "1"  # E: Statement is unreachable  [unreachable]
        else:
            self.x = "2"  # E: Cannot determine type of "x" [has-type]

If an instance attribute is first set in an unreachable block, future attempts to set or read it fail with [has-type].

[A5rocks]

Duplicate of #9709