TypeGuard on self does not narrow associated TypeVars

I have a class which is generic and has some runtime marker for determining the underlying type. I would like to use that marker to guide MyPy in validating the implementation of the class. So if I have a TypeGuard which confirms the current object is generic in A (that is, TypeVar AB_T is bound to A), then it should be safe for the method which is annotated as returning an AB_T to return an A in that context.

What I'm finding instead is that the TypeGuard correctly tells MyPy that the object which knows itself to be Gen[AB_T] is specifically a Gen[A], but it does not make the link that AB_T is A. As such, it refuses to return an A.

I've tested the following example with MyPy 0.991 on Python 3.10

from typing import Generic, Literal, overload, TypeVar
from typing_extensions import TypeGuard

class A: ...
class B: ...
AB_T = TypeVar("AB_T", A, B)

class Gen(Generic[AB_T]):
    # These two overloads ensure that the marker matches the generic type
    @overload
    def __init__(self: "Gen[A]", should_be_A: Literal[True]) -> None:
        ...

    @overload
    def __init__(self: "Gen[B]", should_be_A: Literal[False]) -> None:
        ...

    def __init__(self: "Gen[AB_T]", should_be_A: bool) -> None:
        self._should_be_A = should_be_A

    def res(self) -> AB_T:
        # This should return A() for a Gen[A] and B() for a Gen[B]
        if Gen.gives_A(self): # This calling convention seems to be needed to narrow self
            reveal_type(self) # and it now correctly understands that self is a Gen[A]
            return A() # But this still complains about the scenario where self is a Gen[B]
        elif Gen.gives_B(self):
            reveal_type(self)
            res: AB_T = B() # This also complains about the scenario where AB_T is A, even though we know it is B.
            return res
        else:
            assert False

    def gives_A(self) -> "TypeGuard[Gen[A]]":
        return self._should_be_A

    def gives_B(self) -> "TypeGuard[Gen[B]]":
        return not self._should_be_A

Actual Behavior

generic_specialisation.py:26: note: Revealed type is "generic_specialisation.Gen[generic_specialisation.A]"
generic_specialisation.py:27: error: Incompatible return value type (got "A", expected "B")  [return-value]
generic_specialisation.py:29: note: Revealed type is "generic_specialisation.Gen[generic_specialisation.B]"
generic_specialisation.py:30: error: Incompatible types in assignment (expression has type "B", variable has type "A")  [assignment]

Inizia eseguendo con MyPy l’esempio fornito generic_specialisation.py e confronta i tipi rivelati e gli errori. Verifica in che modo i TypeGuards Gen.gives_A e Gen.gives_B restringono self e se il TypeVar associato viene preservato. Il lavoro è completato quando l’esempio accetta di restituire A per Gen[A] e B per Gen[B] senza gli errori segnalati.