I would like to perform a default implementation (override) on the existing protocol.

public protocol StreamManagerDelegate:URLSessionDataDelegate{
  func streamManager (didReceive data:Data)
}

extensionStreamManagerDelegate{
  funcurlSession(_session:URLSession,
                  dataTask —URLSessionDataTask,
                  DidReceive data:Data) {
    streamManager (didReceive:data)
  }
}

class Stream:NSObject, StreamManagerDelegate {
  func streamManager(didReceive data:Data){
    print(String(data:data, encoding:.utf8)!)
  }
  
  func start(){
     let config = URLSessionConfiguration.default
     let session —URLSession=.init(configuration:config,
                                     delete —self,
                                     deleteQueue:nil)
  }
}

protocolSomeTaskDelegate{
  func someTask (didRecieve data: String)
}

structureSomeTask {
  let delete —SomeTaskDelegate
}

protocolCustomTaskDelegate:SomeTaskDelegate{
  func printData(_value:String)
}

extensionCustomTaskDelegate {
  func someTask (didRecieve data: String) {
    printData(data)
  }
}

structure main:CustomTaskDelegate {
  func printData(_value:String){
    print(value+value)
  }
  
  value —SomeTask!
  
  init(){
    self.value=SomeTask (delegate:self)
  }
}


main().value.delegate.someTask(didRecieve: "1000")