如何使Array <string> Hashable？

Question

我试图让Array<String>符合Hashable，在Swift 3预览版6中：

extension Array: Hashable where Element: Hashable {
    public var hashValue: Int {
        var hashValue = 0
        for element in self {
            hashValue ^= element.hashValue
        }
        return hashValue
    }
}

var data = Dictionary<Array<String>, Any>()

但是我收到以下错误并且不明白为什么：

ERROR at line 4, col 1: extension of type 'Array' with constraints cannot have an inheritance clause
extension Array: Hashable where Element: Hashable {
^    

如何使阵列可以缓存？