title: Why self is deallocated when calling [unowned self]
body: <p>I'm trying write a simple closure as completion handler, and inside the closure set the text value of a textbox:</p>
  
  <pre><code>class ViewController: UIViewController {
  
      @IBOutlet var textArea : UITextView
  
      let sessionConfig = NSURLSessionConfiguration.defaultSessionConfiguration()
  
      let session:NSURLSession?
  
  
      init(coder aDecoder: NSCoder!)  {
          super.init(coder: aDecoder)
  
          session = NSURLSession(configuration: sessionConfig)
      }
  
  
      override func viewDidLoad() {
          super.viewDidLoad()
          // Do any additional setup after loading the view, typically from a nib.
      }
  
      @IBAction func btnSendRequestTapped(sender : AnyObject) {
  
          let url:NSURL  = NSURL(string: "https://www.google.com")
  
          let sessionTask:NSURLSessionTask =
          session!.dataTaskWithURL(url, completionHandler: {
              [unowned self]
              (data:NSData!,response:NSURLResponse!,error:NSError!) -&gt; Void in
              let st:String = NSString(data: data,encoding: NSUTF8StringEncoding)
  
              println("\(st)")
  
              NSOperationQueue.mainQueue().addOperationWithBlock({
                  () -&gt; Void in
                  self.textArea!.text = st
                  })
              })
  
          sessionTask.resume()
      }
  }
  </code></pre>
  
  <p>but on the line where I've defined [unowned self], I'm getting <code>EXC_BREAKPOINT(code=EXC_I386_BPT,subcode=0x0)</code>, and it's showing some assembly code as follow:</p>
  
  <pre><code>libswift_stdlib_core.dylib`_swift_abortRetainUnowned:
  0x1001bb980:  pushq  %rbp
  0x1001bb981:  movq   %rsp, %rbp
  0x1001bb984:  leaq   0x176a7(%rip), %rax       ; "attempted to retain deallocated object"
  0x1001bb98b:  movq   %rax, 0x792ce(%rip)       ; gCRAnnotations + 8
  0x1001bb992:  int3   
  0x1001bb993:  nopw   %cs:(%rax,%rax)
  </code></pre>
  
  <p>I'm not sure what I've done wrong in here, based on the documentation. I've updated the question to contains the whole class.
  Also I've updated the question to update the text property of TextView on the main thread</p>