Narrowing of generic types from a method does not work

[pelme]

To Reproduce
Using a class with a generic type that is returned in a method as a type[T] does not work properly in isinstance narrowing checks:

https://mypy-play.net/?mypy=latest&python=3.12&gist=dd509c91ecf07303e62a00d2d6646345

Expected Behavior
mypy should narrow the union and no error should be emitted.

• ty does the right thing: https://play.ty.dev/2badea13-87aa-46d0-b415-28c98c54b6ac
• pyright has a similar issue to mypy

Actual Behavior
mypy does not narrow the type correctly:

main.py:27: error: Item "A" of "A | B" has no attribute "b_only"  [union-attr]

[A5rocks]

It would be valid for get_type to return a type that is a subtype of A, which means the type should still be A | B.

[pelme]

B is not a subtype of A?

added reveal_type that shows that mypy does narrow it to A in the if clause but then "forgets" about it after the return:

https://mypy-play.net/?mypy=latest&python=3.12&gist=8f49bd1a57b7b28f72181038124ba563

[A5rocks]

This is about narrowing in the else branch, not the if. In this case, if get_type returned a subtype something that is an instance of A would go to the else branch and not return.

[pelme]

how is that different from this example where A is directly passed to the isinstance check(not via the generic type)? https://mypy-play.net/?mypy=latest&python=3.12&gist=d3b26e9007b44af8753c85d6eed2ea42

[A5rocks]

Because type[A] is covariant, so could be any subtype of A.

[A5rocks]

Maybe to make things more concrete:

class A:
    ...
class B:
    ...

def f(x: A | B, y: type[A]):
    if isinstance(x, y):
        return
    
    checked: B = x  # obviously mypy shouldn't allow this

[carljm]

This was a bug in ty, it will be fixed by astral-sh/ruff#23598 - thanks @pelme and @A5rocks for flagging it!

[pelme]

Thanks for looking into this and explaining my misunderstanding.

I have refactored my own code which triggered this to use another approach instead.

I am glad my confusion of this could be helpful and in the end improve ty. 😆