Swift Constraints on extension?

type-constraints.swift

enum Sum<L,R> {
    case Left(@auto_closure () -> L)
    case Right(@auto_closure () -> R)
}

protocol GShow {
    func gshow() -> String
}

extension String : GShow {
    func gshow() -> String {
        return self
    }
}

func show<L : GShow, R : GShow>(x : Sum<L,R>) -> String {
    switch x {
    case .Left(let l):
        return l().gshow()
    case .Right(let r):
        return r().gshow()
    }
}

/* How to do: */

extension Sum : GShow {
    func gshow() -> String {
        let s = show(self) // This line gives an error
        return s
    }
}