OjbC起因かとも思い試しにLinux版でも試すとBAD_ACCESSでますね

class Object {} protocol SomeChild where Self : Object { func doSomething() } extension SomeChild { func doSomething() { print(self) } } class ChildObject: Object, SomeChild {} class ParentObject: Object { var children: [SomeChild] = [ChildObject()] var child: SomeChild = ChildObject() } let parent = ParentObject() parent.child.doSomething() parent.children.forEach { $0.doSomething() }