Generic parameter 'U' cannot be bound to non-@objc protocol type 'SceneObject'

gistfile1.swift

extension Array {
    mutating func removeObject<U: Equatable>(object: U) {
        var index: Int?
        for (idx, objectToCompare) in enumerate(self) {
            if let to = objectToCompare as? U {
                if object == to {
                    index = idx
                }
            }
        }
        
        if(index != nil) {
            self.removeAtIndex(index!)
        }
    }
}

protocol SceneObject {
    var position : CGPoint { get set }
    var anchorPoint : CGPoint { get set }
}


class Scene {
    
    var sceneObjects : [SceneObject] = Array()
    
    func addSceneObject( anObject : SceneObject) {
        sceneObjects.append(anObject)
    }
    
    func removeSceneSceneObject(anObject : SceneObject) {
        sceneObjects.removeObject(anObject) // <-- This failed to compile with the error "Generic parameter 'U' cannot be bound to non-@objc protocol type 'SceneObject'"
    }
    
}