Swift protocol extensions

Maths.swift

protocol Raisable {
    func raise(exponent : Self) -> Self
}

extension Raisable where Self : SignedNumberType {
    func raise(exponent : Self) {
        return pow(self, exponent)
    }
}

protocol Squarable : Raisable {
    func squared() -> Self
}

extension Squarable {
    func squared() -> Self {
        return self.raise(2)
    }
}

Compiler error around overloading the pow function.

I'm attempting to give any number the ability to raised to an arbitrary power (specifically, 2).

Do I have to cast to Double in order to use pow?

http://stackoverflow.com/questions/31702831/swift-2-protocol-extension-using-pow/31703542#31703542