`issubclass()` assertions not narrowing types

Consider the following (contrived) code:

from typing import Type

def untyped_function():
    return int

def foo() -> Type[int]:
    x = untyped_function()
    assert issubclass(x, int)
    return x

Running mypy on this with the --warn-return-any option produces the error "Returning Any from function declared to return "Type[int]"", even though the assert issubclass should convince mypy that x is of type Type[int] and not an Any.

Your Environment

• Mypy version used: 0.902
• Mypy command-line flags: --warn-return-any
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: Python 3.9.5
• Operating system and version: macOS 11.3.1

Start by running the minimal example with mypy 0.902 and --warn-return-any, then trace the type-narrowing logic for assert issubclass(x, int). Add a regression test for the shown case; done means mypy accepts the return as Type[int] without the Returning Any error.