No changes intended, but the existing rules are phrased based
on assumptions that are no longer true with extension types.
Adds more restrictions on when the rules apply, to rule out
things that previously couldn't happen.

It's a change, and it may require inserting a new dynamic
downcast, which the current semantics does not.
(We should decide whether we want this to act like real
"overloading", and if so, how to resolve, or if we should
just have rules that are "good enough" for most common
use-cases.)