部分应用转发器进行重新抽象 thunk helper from @callee_unowned

问：

我有以下 Swift 函数：

func refresh(optionalHandler: @escaping (() -> Void) = {}) {
  afnetworking.get(someURL, parameters: nil, successHandler: {
    (task: URLSessionDataTask, object: Any) in
      print(task)
      optionalHandler() // This crashes with EXC_BAD_ACCESS
  })
}

不知道这里发生了什么......完整的错误是：

optionalHandler () -> ()    0x000000010018baac MyApp`partial apply forwarder for reabstraction thunk helper from @callee_unowned @convention(block) () -> () to @callee_owned () -> () at MyViewController.swift