protocol EntityCreator: class {
    func createEntity<EntityType: EntityClass & EntityCreatorInitializable>(type: EntityType.Type, _ initialize: @escaping (EntityType) -> Void)
}

protocol EntityCreatorInitializable {
    init(entityCreator: EntityCreator)
}

class EntityClass: NSObject { }

class MyClass: EntityClass, EntityCreatorInitializable {
    required init(entityCreator: EntityCreator) {
        super.init()
        // use the entityCreator
    }
}

// On the entity creator implementation :

class EntityCreatorImplementation: EntityCreator {
    func createEntity<EntityType: EntityClass & EntityCreatorInitializable>(type: EntityType.Type, _ initialize: @escaping (EntityType) -> Void) {
        // This creation doesn't work...
        let newEntity = EntityType(entityCreator: self)
        initialize(newEntity)
        self.toAdd.insert(newEntity)
    }
}