Generics handling for protocol return value

generic-protocol.swift

// In response to: http://inessential.com/2015/06/21/swift_protocols_question

import Foundation

protocol Value: Equatable {
}

protocol Smashable {
    func valueBySmashing​OtherValue​<T: Value, U: Value>(value: T) -> U
}

struct Bar : Value {
    
}

struct Foo : Value {
    func valueBySmashing​OtherValue​<T : Value>(value: T) -> Bar {
        return Bar()
    }
}

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

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


let f = Foo()
let b = f.valueBySmashing​OtherValue​(f)
print("bar \(b)")

The issue here is that the argument and return value for valueBySmashing aren't necessarily the same type. The only constraint here is that both types should conform to Value. So the function needs to be parameterized with both T and U, both of which are marked as conforming to Value. That way, the type system can determine that b is of type Bar on line 32, without explicit variable typing.