type(var) is type(other) not considered enough #11952

Bug Report

Hi,

First of all, thanks a lot for your work on mypy and the Python ecosystem in general!

I am sorry if this has been reported before (I would imagine it probably has, but searching for things like "type" and "is" is, well, difficult). It seems that mypy does not realize that type(a) is type(b) is at least as strong a guarantee (well, actually, stronger) than isinstance(a, type(b)).

To Reproduce

(Write your steps here:)

1. Run mypy --strict on the attached program (saving it with a .py extension beforehand).
2. Watch it complain about the _bad functions returning Any instead of the declared type.
3. Run the program to see that it, indeed, works.

Expected Behavior

No diagnostic messages from mypy.

Actual Behavior

Complaints about functions that check that type(result) is type(value).

Your Environment

Checked both with Python 3.9 + mypy 0.921 and Python 3.8 + mypy 0.910 on Debian testing. In both cases mypy was invoked with --strict --python-version 3.8 and no additional configuration.

Thanks again, and keep up the great work!

G'luck,
Peter
mypytype.txt