ErrorBox for Xcode 8b4 and Xcode 8b5

ErrorBox.swift

import Foundation

/// Example error
enum MyError : Error {
    case error1
}


/// Boxes error in a struct
struct ErrorBox<BoxedError: Error> {
    let error: BoxedError
    init(_ error: BoxedError) {
        self.error = error
    }
}


/// Example NSObject subclass containing an error property
class MyClass : NSObject {
    // Uncommenting this line will cause a crash
    // var error: MyError?

    // Workaround is to box the error in an intermediate struct and access the error via the box.
    var errorBox: ErrorBox<MyError>?
}


let object = MyClass()
object.errorBox = ErrorBox(.error1)
if let error = object.errorBox?.error {
    print(error)
}

This is no longer necessary as of Xcode 8b6.