Incorrect inferred type for attribute with two partial types

Unexpected type is inferred for attribute x in this example:

class C:
    def __init__(self) -> None:
        if int():
            self.x = None
            return
        self.x = []
        self.x.append("x")

reveal_type(C().x)  # "list[str]" -- but should be "list[str] | None"

The inferred type should be list[str] | None, or alternatively mypy could require a type annotation for x, if this is too tricky to infer.

This should be tested with and without --allow-redefinition-new, since it may make a difference.

Start by reproducing the provided class C example in mypy, checking the revealed type with and without --allow-redefinition-new. Trace the inference behavior for the two assignments to x and add a regression test covering both modes; done means mypy reports list[str] | None or consistently requires an annotation.