@preconcurrency attribute on conformance to 'AProtocol' has no effect

Using Swift
, ,
1

When adopting Swift 6, it's common to have cases when @MainActor class needs to conform to a protocol that has no explicit isolation:

protocol A {
    func foo()
}

class VC: UIViewController, A {
    func foo() {} // Main actor-isolated instance method 'foo()' cannot be used to satisfy nonisolated protocol requirement
}

This case is covered in the official documentation. One of the solutions is to conform to a protocol with the @preconcurrency attribute.

But when we add new protocol B that inherits protocol A, the warning appears:


protocol A {
    func foo()
}

protocol B: A {}

class VC: UIViewController, @preconcurrency B { // @preconcurrency attribute on conformance to 'B' has no effect
    func foo() {}
}

Why is that? Is it expected behavior or a bug?
When we add a function to protocol B, the warning disappears:

protocol A {
    func foo()
}

protocol B: A {
    func bar()
}

class VC: UIViewController, @preconcurrency B {
    func foo() {}
    func bar() {}
}

How do you find out in which cases adding @preconcurrency to protocol conformance has no effect?

1 Like
2

I noticed the same, and it could be a bug in this case but seems to be related to whether the protocol body is empty. A workaround is to make the A conformance explicit and attach @preconcurrency there:

protocol A {
    func foo()
}

protocol B: A {}

class VC: UIViewController, @preconcurrency A, B {
    func foo() {}
}