Incompatible return type [7]: Expected `Variable[T]` but got `Variable[R]`.Pyre

Pyre type fails because it assumes an incorrect return type R when we are returning T. It happens when we use a derived class in another class and access a field that is generic typed from the parent class.

Following is the toy code for which it is complaining at the last line.

T = TypeVar("T")
R = TypeVar("R")

@dataclass(frozen=True)
class A(Generic[T, R]):
    t: T
    r: R

class B(A[T, R]):
    ...

@dataclass(frozen=True)
class X(Generic[T, R]):
    a: A[T, R]
    b: B[T, R]

    def getfromA(self) -> T:
        return self.a.t

    def getfromB(self) -> T:
        # pyre-fixe[7] Incompatible return type [7]: Expected `Variable[T]` but got `Variable[R]`.Pyre
        return self.b.t

Notice that it works correctly when using A, the base class with generic. But when using with B, the derived class, it inverts the types.

Am I missing something or am I hitting some known pyre bug?

Thanks