dataclasses.is_dataclass() assumed always true when passed type

[efroemling]

Bug Report

In the latest 1.1.1 build of Mypy I've started getting various errors such as 'code is unreachable' or 'statement is always true' around certain uses of dataclasses.is_dataclass(). It seems to happen when the argument to is_dataclass() is a type.

To Reproduce

Here is a playground demonstrating the issue:
https://mypy-play.net/?mypy=master&python=3.11&flags=warn-unreachable&gist=5001f76768535e01b95e69dd2c47a916

If you run it with mypy 1.0.0 it passes, but with mypy master branch it says the 'not-dataclass' path cannot be reached.

The code:

import dataclasses
from typing import Any


def what_is_it(obj: type) -> str:
    if dataclasses.is_dataclass(obj):
        return 'dataclass'

    return 'not-dataclass'

Expected Behavior

Mypy should not assume is_dataclass() always returns True when passed a type.

Actual Behavior

Mypy assumes is_dataclass() always return True when passed a type.

Your Environment

Seeing the error in Mypy 1.1.1 from home-brew on a Mac. 1.1.1 does not seem to be available on the playground but the 'mypy-master-branch' currently exhibits the error.

[AlexWaygood]

Apologies for the regression. This has already been fixed in 4ca0c48, but the fix didn't make it into mypy 1.1.1. It's possible mypy will be willing to make a patch release for this.

[srittau]

I'm going to close this here as this seems to be a mypy bug, not related to typeshed.