ProtocolExtensions.swift

protocol Protocol {
    func methodInProtocol() -> String
}

extension Protocol {
    func methodInProtocol() -> String {
        return "Protocol"
    }
    func methodOnExtension() -> String {
        return "Protocol"
    }
}

struct Implementer: Protocol {
    func methodInProtocol() -> String {
        return "Implementer"
    }
    func methodOnExtension() -> String {
        return "Implementer"
    }
}

struct Underimplementer: Protocol {
    // does not implement or override anything in Protocol
}

// Calling a method declared in the protocol and implemented in a protocol extension:
// Using actual type, method on Implementer is called, falling back to protocol extension method:
Implementer().methodInProtocol()      // "Implementer"
Underimplementer().methodInProtocol() // "Protocol"

// Using the protocol type, same behavior as above _because the method is in the protocol_:
(Implementer() as Protocol).methodInProtocol()      // "Implementer"
(Underimplementer() as Protocol).methodInProtocol() // "Protocol"


// Calling a method _not_ in the protocol, but on the extension,
// when called on the type, the method on the extension is the fallback:
Implementer().methodOnExtension()      // "Implementer"
Underimplementer().methodOnExtension() // "Protocol"

// Because the method is not declared in the protocol, calling it upon the protocol calls the extension implementation:
(Implementer() as Protocol).methodOnExtension()      // "Protocol"
(Underimplementer() as Protocol).methodOnExtension() // "Protocol"