The `@reparented` was missing and a typealias
was being synthesized unexpectedly, creating
an issue when typechecking the interface later.
There's no fundamental reason why typealiases
cannot be supported to say the same thing as
the same-type requirement, but I think the
same-type requirement is always needed to be
written on the extension, one way or another.
For now I've chosen to only go with an
explicitly-written same-type requirement.
