[PEP 695] Incorrect Variance Computation with `Self | other` return type

In the example below, FloatScalar is contravariant in FloatT, but mypy thinks it is invariant

from typing import Self

class FloatScalar[FloatT]:  # contravariant
    def pow(self, arg: FloatT) -> Self | complex: ...

def upcast(arg: FloatScalar[object]) -> FloatScalar[float]:
    return arg  # ❌️ Incompatible return value type

https://mypy-play.net/?gist=e9e6d34bee57d1c799b71b0f0573175d

If we remove the union in the return type, inference seems to work as expected:

from typing import Self

class FloatScalar[FloatT]:  # contravariant
    def pow(self, arg: FloatT) -> Self: ...

def upcast(arg: FloatScalar[object]) -> FloatScalar[float]:
    return arg  # ✅️

https://mypy-play.net/?gist=b0a994e8dda1b85ce2a4cf26631f843e

Start with the linked mypy-play reproduction and compare the two examples, where adding Self | complex changes the inferred variance of FloatScalar. Trace the variance computation for the PEP 695 generic and add a regression test showing that FloatScalar[object] is accepted as FloatScalar[float] in the union case.