Written as an attempted solution to Brent Simmons's blog post.

Smashable Protocol

// Written by Alexandre Lopoukhine

protocol Value: Equatable {}

protocol Smashable {
    typealias SmashValue
    func valueBySmashingOtherValue<T: Value>(value: T) -> SmashValue
}

struct Bar: Value {
}

func ==(lhs: Bar, rhs: Bar) -> Bool {
    return true
}

struct Foo {
    typealias SmashValue = Bar
    
    func valueBySmashingOtherValue<T: Value>(value: T) -> Bar {
        return Bar()
    }
}