MainActor isolated protocol affects on child class but not superclass

Hey all,

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

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

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

protocol MyProto {}

class Child: Base {
    override func foo() {

    func fooIsolated() {

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?