`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

Inizia eseguendo l’esempio minimo con mypy 0.902 e --warn-return-any, quindi traccia la logica di restringimento dei tipi per assert issubclass(x, int). Aggiungi un test di regressione per il caso mostrato; il lavoro è completato quando mypy accetta il valore restituito come Type[int] senza l’errore Returning Any.