diamond problem

sample.swift

protocol StringConvertible {}
extension StringConvertible {
    func toString() -> String {
        return "StringConvertible: \(self)"
    }
}

protocol StringType {}
extension StringType {
    func toString() -> String {
        return "StringType: \(self)"
    }
}

class User : StringConvertible, StringType {}

let userA:StringConvertible = User()
print(userA.toString()) // StringConvertible: User

let userB:StringType = User()
print(userB.toString()) // StringType: User