Declarations from extensions cannot be overridden yet

我已经阅读了多篇重新评论此问题的帖子.但他们都没有接受下面描述的情景：

class BaseCell: UITableViewCell
{
    //Some code here...
}

extension BaseCell
{
    func isValid() -> String?
    {
        //Some code here...
    }
}

class SampleCell: BaseCell
{
    //Some code here...

    override func isValid() -> String? //ERROR..!!!
    {
        //Some code here...
    }
}

根据Apple的说法,

Extensions can add new functionality to a type,but they cannot override existing functionality.

但在上面的场景中,我没有在扩展中覆盖方法isValid().它在SampleCell类定义本身中被重写.不过,它正在给出错误.