# MainActor isolated protocol affects on child class but not superclass

**URL:** <https://forums.swift.org/t/mainactor-isolated-protocol-affects-on-child-class-but-not-superclass/71661>\
**Category:** Using Swift\
**Tags:** concurrency, protocols, mainactor\
**Created:** [May 7, 2024, 12:05pm UTC](https://forums.swift.org/t/mainactor-isolated-protocol-affects-on-child-class-but-not-superclass/71661 "2024-05-07T12:05:28Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![hamtiko](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/hamtiko/32/35225_2.png) [@hamtiko](https://forums.swift.org/u/hamtiko)\
**Post date:** [May 7, 2024, 12:05pm UTC](https://forums.swift.org/t/mainactor-isolated-protocol-affects-on-child-class-but-not-superclass/71661/1 "2024-05-07T12:05:28Z")

</div>

Hey all,

I have faced following strange behavior of the swift compiler (Xcode 15.3):

```swift
class UseCase {
    func test() {
        Base().foo()
        Child().foo()
        Child().fooIsolated() // Error: Call to main actor-isolated instance method 'fooIsolated()' in a synchronous nonisolated context
    }
}

class Base {
    func foo() {
        print("Base")
    }
}
extension Base: MyProto {}

@MainActor
protocol MyProto {}

class Child: Base {
    override func foo() {
        print("Child")
    }

    func fooIsolated() {
        print("aa")
    }
}

```

So it seems to me that @MainActor isolated protocol applies to the Childs of the class which is conforming that protocol, but not to that base class. So as you can see from the code I have `Base` class and `MyProto` protocol which is marked as `MainActor` isolated. The `Base` class conforms `MyProto` protocol with extension. There is also a `Child` class which inherits from `Base` and overrides the `foo` function and adds its own `fooIsolated` function. In the `UseCase` class where we have non isolated context, the compiler gives an error for calling `fooIsolated` function of `Child` but not for `foo` functions for both `Child` and `Base` classes. From this I can say that the `fooIsolated` function was isolated but the interface inherited from `Base` and the superclass itself is not isolated.

Another strange behavior I noticed regarding the warning which we should get in this cases "Main actor-isolated class 'Child' has different actor isolation from nonisolated superclass 'Base'; this is an error in Swift 6". this warning is not fired but it should from my understanding, and it fires only when I explicitly mark `Child` as `MainActor` or the `Child` class was defined in different package than `Base` class.

Any ideas on this?

Thanks

---

<div class="post-metadata">

**Author:** ![hamtiko](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/hamtiko/32/35225_2.png) [@hamtiko](https://forums.swift.org/u/hamtiko)\
**Post date:** [June 11, 2024, 12:01am UTC](https://forums.swift.org/t/mainactor-isolated-protocol-affects-on-child-class-but-not-superclass/71661/2 "2024-06-11T00:01:32Z")

</div>

Same issue happens on Xcode 15.4 even with brand new Xcode 16 and Swift 6.

---

<div class="post-metadata">

**Author:** ![bjhomer](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/bjhomer/32/313_2.png) [@bjhomer](https://forums.swift.org/u/bjhomer)\
**Post date:** [June 11, 2024, 3:41am UTC](https://forums.swift.org/t/mainactor-isolated-protocol-affects-on-child-class-but-not-superclass/71661/3 "2024-06-11T03:41:54Z")

</div>

Overridden functions _must_ inherit the isolation of the function they are overriding, so this expected behavior. If Child.foo() were isolated, then the compiler would somehow have to prevent you from calling `base.foo()` in a non-isolated context _only_ when base is actually an instance of Child. But since the compiler cannot know that at compile time, the only safe thing it can do is enforce that the isolation of the function is the same as the parent it is overriding.

---

<div class="post-metadata">

**Author:** ![hamtiko](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/hamtiko/32/35225_2.png) [@hamtiko](https://forums.swift.org/u/hamtiko)\
**Post date:** [June 11, 2024, 5:25am UTC](https://forums.swift.org/t/mainactor-isolated-protocol-affects-on-child-class-but-not-superclass/71661/4 "2024-06-11T05:25:45Z")

</div>

That makes sense when you have an actor-isolated child whose parent is not isolated. However, that’s not the issue I’m referring to. The problem here is that the main actor-isolated protocol conformance affects the child but not the base. I believe it should not affect either of them since the conformance is added through an extension. Alternatively, if it affects the child, the parent should be affected as well.