'Subclass of "A" and "B" cannot exist: would have incompatible method signatures' is usually wrong

Mypy reports Subclass of "A" and "B" cannot exist: would have incompatible method signatures in many cases if given x: A, you run isinstance(x, B). However, this is often (always?) incorrect, as mypy does not account for subclasses that use method signatures that are subtypes of both classes.

Example (playground):

from typing import Never

class A:
    def f(self) -> int: return 0
class B:
    def f(self) -> str: return ""

class X(A, B):
    def f(self) -> Never: raise Exception

x: A = X()
if isinstance(x, B):  # E: Subclass of "A" and "B" cannot exist: would have incompatible method signatures
    reveal_type(x)    # E: Statement is unreachable
else:
    reveal_type(x)    # N: Revealed type is "__main__.A"

Here, mypy accepts the definition of a class X that is a subclass of A and B, yet also says that such a subclass cannot exist.

Any callable type is a subclass of (*args: object, **kwargs: object) -> Never, so incompatible method signatures cannot be a reason that a common subclass cannot exist.

It gets a little trickier with attributes. However, mypy accepts this class and then again claims it cannot exist (playground):

from typing import Never

class A:
    x: int
class B:
    x: str

class X(A, B):
    @property
    def x(self) -> Never: raise Exception
    @x.setter
    def x(self, x: int | str) -> None: pass

x: A = X()
if isinstance(x, B):  # E: Subclass of "A" and "B" cannot exist: would have incompatible method signatures
    reveal_type(x)    # E: Statement is unreachable
else:
    reveal_type(x)    # N: Revealed type is "__main__.A"

(Note though that pyright rejects this form of inheritance; possibly mypy should reject this too in the future if we revise how we treat inheritance of attributes.)

Reproduziere die aufrufbaren und Attributbeispiele in den verlinkten mypy-play-Playgrounds und verfolge dann die Typverengung und die Prüfungen auf inkompatible Signaturen, die die Diagnose erzeugen. Ermittle, ob gültige gemeinsame Unterklassen den Fehler und die Warnung wegen Unerreichbarkeit unterdrücken sollten, und füge eine Regressionstestabdeckung hinzu, die das beabsichtigte Verhalten demonstriert.