Self-type doesn't seem to work for properties #2402

I found this when trying to use self-type for pathlib (see PR python/typeshed#658). There are a few properties that have type PurePath and are repeated in the Path subclass. The stub uses plain attributes for these, but the implementation has properties, so I want to replace them with properties. However this doesn't seem to work. In particular I'm trying to add this to class PurePath:

    @property
    def parents(self: _P) -> Sequence[_P]: ...
    @property
    def parent(self: _P) -> _P: ...

Now I have an example class:

class MyPath(pathlib.PosixPath):
    def parity(self):
        return len(str(self)) % 2

root = MyPath('/')
reveal_type(root.parent)

This reveals MyPath as the type but also gives an error:

__tmp__.py:14: error: Revealed type is '__tmp__.MyPath*'
__tmp__.py:14: error: Invalid method type

If I remove the self-type, e.g.

    @property
    def parents(self) -> Sequence[PurePath]: ...
    @property
    def parent(self) -> PurePath: ...

the error goes away but the revealed type reverts back to (of course) PurePath.

@elazarg thoughts?