issubclass doesn't narrow down types

Bug Report

issubclass appears to not narrow down possible types.

To Reproduce

from typing import Any, Mapping, MutableMapping


def test_mutablemapping(obj: Mapping[Any, Any]) -> MutableMapping[Any, Any]:
    if issubclass(type(obj), MutableMapping):
        return obj  # error: Incompatible types in assignment (expression has type "Mapping[Any, Any]", variable has type "MutableMapping[Any, Any]")
    else:
        return dict(obj)

Expected Behavior

To narrow down in similar fashion to isinstance:

from typing import Any, Mapping, MutableMapping


def test_mutablemapping2(obj: Mapping[Any, Any]) -> MutableMapping[Any, Any]:
    if isinstance(obj, MutableMapping):
        return obj
    else:
        return dict(obj)

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: default
• Mypy configuration options from mypy.ini (and other config files): default
• Python version used: 3.8.8
• Operating system and version: Windows 10

Beginne mit dem test_mutablemapping-Reproducer und vergleiche dessen issubclass(type(obj), MutableMapping)-Zweig mit dem isinstance-Beispiel im Issue. Verfolge, wie mypy die Typeneingrenzung für issubclass behandelt, und füge einen Regressionstest hinzu, der zeigt, dass der Zweig MutableMapping[Any, Any] ohne Fehler zurückgibt.