[Python] Mypy reports parameter whose type is a TypeVar bound to the same type as its default...

When annotating a function parameter with a bound TypeVar, giving it a default value results in the parameter having a union type between the TypeVar and the default value type, even though the default value is of the TypeVar type.

Example:

class A:
pass

class B(A):
pass

Instance = TypeVar("Instance", bound=A)

def get_instance(cls: type[Instance] = A) -> Instance:
return cls()


Running mypy yields the following error: error: Incompatible default for argument "cls" (default has type "Type[A]", argument has type "Type[Instance]").

reveal_type is correct in both cases:

instance_a = get_instance(cls=A)
reveal_type(instance_a) # note: Revealed type is "A"

instance_b = get_instance(cls=B)
reveal_type(instance_b) # note: Revealed type is "B"


How do I correctly annotate get_instance so that I can keep the default argument?

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