I believe it was @jrose who mentioned at some point fully banning such conformances. It's probably too code-breaking at this point, but the reality is that such code is liable to break at any time. Less so when the type and protocol are owned by two disparate vendors but definitely the case if you try to conform Apple's types to Apple's protocols, or Swift's types to Swift's protocols.
Your example here would be a role for cross-import overlays. Bob would then vend the conformance.