Result<T,U> working in Xcode 6.3

MyPlayground.swift

//: Playground - noun: a place where people can play

import Darwin

class Box<T> {
    let contents: T

    init(_ contents: T) { self.contents = contents }
}

enum Result<T, U> {
    case _Success(Box<T>)
    case _Failure(Box<U>)

    static func Success(t: T) -> Result { return ._Success(Box(t)) }
    static func Failure(u: U) -> Result { return ._Failure(Box(u)) }

    func onSuccess(action: (T) -> ()) -> Result {
        switch self {
            case let ._Success(box):
                action(box.contents)
            default: let nop = 0
        }
        return self
    }

    func onFailure(action: (U) -> ()) -> Result {
        switch self {
            case let ._Failure(box):
                action(box.contents)
            default: let nop = 0
        }
        return self
    }
}

func doThing() -> Result<String, Int> {
    return arc4random_uniform(2) == 0 ? .Success("Hello world") : .Failure(42)
}

doThing().onSuccess { message in println("Success! \(message)") }
         .onFailure { message in println("Failure! \(message)") }

Playground execution failed: OptionalStuff.playground:12:1: warning: code after 'return' will never be executed
}
^
integer overflows when converted from 'Builtin.Int32' to 'Builtin.Int8'OptionalStuff.playground:5:6: error: unimplemented IR generation feature non-fixed multi-payload enum layout
enum Result<T, U> {
^

See older revision for broken version. Regarding the non-fixed multi-payload, the ResultBox type is used.