Demonstrate an EXC_BAD_ACCESS error when attempting to use optional chaining on a weak delegate variable.

WeakDelegate.swift

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

import Cocoa

@class_protocol protocol DownloaderDelegate {
    func finishedDownloading(fileName: String)
}

class Downloader {
    weak var delegate: DownloaderDelegate?
    func download(fileName: String) {
        delegate?.finishedDownloading(fileName)
    }
}

class Owner: DownloaderDelegate {
    func finishedDownloading(fileName: String) {
        println("All done! \(fileName)")
    }
}

let owner = Owner()
let downloader = Downloader()
downloader.delegate = owner
downloader.download("lolcat.png")

/*
Playground execution failed: error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=2, address=0x1074df490).
The process has been left at the point where it was interrupted, use "thread return -x" to return to the state before expression evaluation.
* thread #1: tid = 0xa5cc5e, 0x00000001074df490, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x1074df490)
  * frame #0: 0x00000001074df490
    frame #1: 0x00000001074d78c1
    frame #2: 0x00000001072eb6b0 libswift_stdlib_core.dylib`_TtCSs13_NSSwiftArray + 128
    frame #3: 0x00000001072eb6d8 libswift_stdlib_core.dylib`SwiftObject + 40
*/

Error goes away when declaring the delegate as strong. This feels like a bug to me.

Also notable: Replacing @class_protocol with @objc on the protocol declaration clears up the issue entirely, as per this StackOverflow comment.