Method of MainActor-isolated value not called on the main actor

[gwendal.roue (Gwendal Roué)]

@gwendal.roue

To go further: if Region-Based-Isolation would allow, in some circumstances, to accept such code, then consistency would strongly suggest that send() is always called on the main actor, in order to avoid surprises and subtle/unexpected behavior changes.

There we are: a crasher that involves sending. No compiler warning, no compiler error, but a runtime crash:

/// This class takes place of CurrentValueSubject.
/// It is not sendable, and in our particular case we
/// want to make sure its `send` method is called on the
/// main actor.
class MySubject {
    func send() {
        MainActor.assumeIsolated {
            print("Good: on main thread")
        }
    }
}

@available(*, unavailable)
extension MySubject: Sendable { }

/// A class that builds a MainActor-isolated subject, and calls its `send`
/// method from a non-isolated context.
final class Context: Sendable {
    @MainActor func makeSubject() -> sending MySubject {
        MySubject()
    }
    
    func send() async {
        await makeSubject().send()
    }
}

@main struct App {
    static func main() async {
        let context = Context()
        // Will it crash? Yes it crashes.
        await context.send()
    }
}