Block.swift

import Foundation

typealias CallbackBlock = @convention (block) () -> Void                        
                                                                                
func main() -> CallbackBlock {                                                  
  let handler: CallbackBlock = { print("yes") }                                 
  let x = NSMutableDictionary()                                                 
  x["foo"] = handler                                                            
  let handler2 = x["foo"] as! CallbackBlock                                     
  return handler2                                                               
}                                                                               
                                                                                
let _ = main()

func main2() -> CallbackBlock {                                                  
  let handler: CallbackBlock = { print("yes") }                                 
  let x = NSMutableDictionary()                                                 
  x["foo"] = handler                                                            
  let handler2 = unsafeBitCast(x["foo"] as AnyObject, to: CallbackBlock.self)   
  return handler2                                                               
}                                                                               
                                                                                
let _ = main2()