How to understand the join function in this string extension?

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in Xcode I found this extension String

:

extension String {
    func join<S : SequenceType where String == String>(elements: S) -> String
}

      

        
        
        
      

    

I expect it to be

extension String {
    func join<S : SequenceType where S.Element == String>(elements: S) -> String
}

      

        
        
        
      

    

because I think the goal should be to ensure the elements are in sequence String

when passing to a function join

.

So my questions are:

• Is the function declaration correct in Xcode 6 GM?
• If so, why does it make sense?