MyPy fails type narrowing on `not issubclass(X, type[T]): raise`

Bug Report

MyPy fails to narrow types using issubclass, and expects an isinstance check which is wrong.

MyPy fails type narrowing on not issubclass(X, type[T]): raise and complains about returning X() later for the function marked as returning T.

To Reproduce

class MySentinel:
    pass

# Works as expected
class OptionalSentinel:
    def __init__(self, value: object) -> None:
        self._value = value

    def get_value[T](self, expected: type[T]) -> Iterator[T]:
        if not isinstance(self._value, expected):
            raise TypeError

        yield self._value

# Reveals bug
class MagicIterator:
    def raw[T](self, expected: type[T]) -> Iterator[T]:
        """Get a list of repositories which accept a specific lint to run."""
        if not issubclass(MySentinel, expected):
            raise ValueError

        # if TYPE_CHECKING:
        #     # an ugly workaround to make mypy happy
        #     if not isinstance(expected, MySentinel):
        #         raise ValueError

        for project in [MySentinel(), MySentinel()]:
            yield project  # error: Incompatible types in "yield" (actual type "MySentinel", expected type "T")  [misc]
```

Expected Behavior

I expected mypy to infer that after an if not issubclass(T, X): raise block X must be subclass T and it should be ok to return X in place of T.

Since I pass a type and want to compare it to a type an isinstance check is not appropriate for MagicIterator, neiter o I think the issubclass check is the most appropriate (I would prefer to check using expected is not T to guarantee that expected is actually exactly T but ìs`i not listed in the list of supported narrowing expressions.

Actual Behavior

Mypy is only happy if I add an isinstance check, which I deem wrong since I dont pass an instance but a type to the function. Thus, if I do not put the incorrect code in a if TYPE_CHECKCING block my program crashes

src/typeTest.py:31: error: Incompatible types in "yield" (actual type "MySentinel", expected type "T")  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.18.2 (compiled: yes)
• Mypy command-line flags: none (mypy src)
• Mypy configuration options from pyproject.toml (and other config files):
• Python version used: 3.13.7 (uv)

** Possibly related **:

• https://github.com/python/mypy/issues/10680 (I think these go hand in hand)
• https://github.com/python/mypy/issues/19529
• https://github.com/python/mypy/issues/9003 (might be related, not sure)
• https://github.com/python/mypy/issues/17728 (similar but seems different)

Commencez par l’exemple MyPy Playground lié et comparez le chemin de narrowing de issubclass qui échoue avec l’exemple isinstance qui fonctionne. Lisez les issues associées #10680, #19529, #9003 et #17728 avant de localiser les tests de narrowing pertinents et l’implémentation ; c’est terminé lorsque l’exemple passe le type-checking sans le workaround d’exécution incorrect.