The best API design is likely to erase the public type to the `any Error` existential …

… but Swift has also the tooling to declare ABI-stable enums without freezing them. Clients are forced to handle unknown cases.

Yeah it could be risky. Might be a feature best used inside the library implementation but not exposed in the public interfaces.

I’m not sure how this would interact with the API compatibility Swift has. Maybe that would help authors.