[Question] Is it possible to declare Protocol with actor inheritance and @MainActor

[fummicc1]

Description

a protocol with actor inheritance can be annotated by @MainActor but it is not possible in my understanding.

• example code

// this protocol can pass compilation though I can't declare actor which conform to `B`.
@MainActor
protocol B: Actor {}

Steps to reproduce

• running above example code and check xcode can compile this code.

Expected behavior

I hope swift will say error against above code.

Environment

• Swift compiler version info: swift 5.7.2
• Xcode version info: Xcode 14.2
• Deployment target: iOS 13

[AnthonyLatsis]

@ktoso Is this protocol indeed useless?

[ktoso]

It's pretty useless yeah.

Only an actor can implement Actor, and @MainActor would mean conflicting isolation. Classes are not and should not be allowed to be : Actor (see also #62632 ).

There's some very weird things one could argue for why this is legal to declare but illegal to implement in any reasonable way. Tbh this might need more discussion pending "delegate actors" re custom executors where we could guarantee that actor MUST be statically on the same executor as the main actor etc... but we don't have such facilities today.

I'm not sure if worth the effort for a diagnostic here, but yeah it is pretty useless to declare such protocol right now