Extension of protocol cannot have inheritance clause

Money.swift

protocol Money {
    var value:  Double { get }
    var inGold: Double { get }
    var symbol: String { get }
}

// Sadly, gives a compiler error.
// "Extension of protocol 'Money' cannot have an inheritance clause"
extension Money: CustomStringConvertible {
    var description: String {
        return "\(value)\(symbol)"
    }
}

Money2.swift

// This works though. Compiles fine.
protocol Money: CustomStringConvertible {
    var value:  Double { get }
    var inGold: Double { get }
    var symbol: String { get }
}

extension Money {
    var description: String {
        return "\(value)\(symbol)"
    }
}

I think you have to let protocol Money inherit from CustomStringConvertible in the first place:

protocol Money: CustomStringConvertible {
    var value:  Double { get }
    var inGold: Double { get }
    var symbol: String { get }
}

and then, add the following protocol extension:

extension Money {
    var description: String {
        return "\(value)\(symbol)"
    }
}

a working example: struct Diamond: Money

I'm not sure whether I'm right or not, so tell me if I'm wrong.