具體類型的錯誤，同時增加約束的協議擴展

Question

從雨燕編程語言指南：

extension Collection where Iterator.Element:TextRepresentable 
{ 
    var textualDescription:String { 
     let itemsAsText = self.map { $0.textualDescription } 
     return "[" + itemsAsText.joined(separator: ", ") + "]" 
    } 
} 

...

let hamsters = [murrayTheHamster, morganTheHamster, mauriceTheHamster] 

print(hamsters.textualDescription) 

此時編譯器告訴我：

不支持使用'TextRepresentable'作爲符合協議'TextRepresentable'的具體類型。

我有點驚訝，因爲代碼是右出的語言指導。這是語言指南中的錯誤，還是我輸入了錯誤的內容？或者也許我的編譯器設置是錯誤的？

Answer 1

工作正常，我和Xcode 8.2版測試版（8C23）：

import UIKit 

protocol TextRepresentable { 
    var textualDescription: String { get } 
} 

extension Collection where Iterator.Element:TextRepresentable 
{ 
    var textualDescription:String { 
     let itemsAsText = self.map { $0.textualDescription } 
     return "[" + itemsAsText.joined(separator: ", ") + "]" 
    } 
} 

struct Hamster { 
    var name: String 
    var textualDescription: String { 
     return "A hamster named \(name)" 
    } 
} 
extension Hamster: TextRepresentable {} 

class ViewController: UIViewController { 

    override func viewDidLoad() { 
     super.viewDidLoad() 

     let murrayTheHamster = Hamster(name: "Murray") 
     let morganTheHamster = Hamster(name: "Morgan") 
     let mauriceTheHamster = Hamster(name: "Maurice") 
     let hamsters = [murrayTheHamster, morganTheHamster, mauriceTheHamster] 

     print(hamsters.textualDescription) 

    } 

    override func didReceiveMemoryWarning() { 
     super.didReceiveMemoryWarning() 
     // Dispose of any resources that can be recreated. 
    } 

}