generic is incorrectly inferred as invariant if class uses `Self` type on an attribute

[DetachHead]

from typing import Self


class Foo[T]:
    instance: Self # commenting out this line makes the error go away

    def foo(self) -> T: ... # type:ignore[empty-body]


class Bar(Foo[int]): ...

# Incompatible types in assignment (expression has type "Bar", variable has type "Foo[object]")  [assignment]
foo: Foo[object] = Bar()

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