Solving http://inessential.com/2015/06/21/swift_protocols_question

gistfile1.swift

// One note before we start: if the inhabitants of Value are a closed set,
// then making Value an enum is going to be a clearer model than using a
// protocol like this. I'm assuming you need to be able to retroactively add
// new members of Value.

// Can't use Equatable directly because of its Self requirement.
protocol HeteroEquatable {
    func isEqualTo(value: HeteroEquatable) -> Bool
}

func ==(lhs: HeteroEquatable, rhs: HeteroEquatable) -> Bool {
    return lhs.isEqualTo(rhs)
}

protocol Value: HeteroEquatable {}

protocol Smashable {
    func valueBySmashingOtherValue(value: Value) -> Value
}

extension Int: Value {
    func isEqualTo(a: HeteroEquatable) -> Bool {
        if let a = a as? Int {
            return self == a
        } else {
            return false
        }
    }
}

struct Test {
    func valueBySmashingOtherValue(value: Value) -> Value {
        return 0 // This is free to return a different adopter of Value
    }
}

let result = Test().valueBySmashingOtherValue(3)
println(result == 3) // can still ==

Okay. No. I don't think it's fixable in Swift currently.

Per @jckarter, Value can't be both Equatable and used as a dynamic type because Equatable requires a Self constraint, but the result of valueBySmashingOtherValue would be type-erased, so you couldn't actually use it as an equatable…

You could get around this by making Value heterogeneously equatable.

(updated to a version with Value heterogeneously equatable)