转换结构失败，并且：...无法转换为

Question

我有一个协议：

protocol Occurrence {
    var isEmpty: Bool { get }
    mutating func addOccurrence(occ: Occurrence) -> Occurrence
    mutating func removeOccurrence(occ: Occurrence) -> Occurrence
}

符合该协议的结构：

struct NonEmptyOccurrence: Occurrence, Printable {

    ...
    private var _occurrence: Int

    var isEmpty: Bool {
        get {
            return false
        }
    }

    ...

    mutating func addOccurrence(other: Occurrence) -> Occurrence {
        if other.isEmpty {
            return self
        } else {
            //error here: Occurrence is not convertible to NonEmptyOccurrence
            _occurrence = _occurrence + (other as NonEmptyOccurrence)._occurrence 
            return self
        }
    }
}

在addOccurrence方法中，我将其添加到评论中时出现错误。我在这里错过了什么？为什么我不能将该实例转换为NonEmptyOccurrence？

Answer 1

尝试类似：

mutating func addOccurrence(other: Occurrence) -> Occurrence {
    if !other.isEmpty, let otherAsNEO = (other as? NonEmptyOccurrence) {
        _occurrence = _occurrence + otherAsNEO._occurrence 
    }
    return self
}

Answer 2

事实证明你可以在Xcode 6 beta3中投射结构，但是你不能使用当前的稳定版本（Xcobe 6.2）。