CustomCollection.scala

import scala.collection.{GenTraversableOnce, LinearSeq, LinearSeqLike, mutable}
import scala.collection.generic.CanBuildFrom

object Message {
  def apply[A](args: A*): Message[A] = new Message(args)

  def newBuilder[A]: mutable.Builder[A, Message[A]] =
    new mutable.ArrayBuffer[A].mapResult(buf => new Message(buf))

  implicit def builderFactory[A]: CanBuildFrom[Any, A, Message[A]] = new CanBuildFrom[Any, A, Message[A]] {
    def apply() = Message.newBuilder[A]
    def apply(a: Any) = apply()
  }
}
class Message[A] private (args: Seq[A])
  extends LinearSeq[A] with LinearSeqLike[A, Message[A]] {

  type Repr = Message[A]

  def apply(idx: Int): A = args(idx)

  def length: Int = args.length

  override def iterator: Iterator[A] = args.iterator

  override def newBuilder: mutable.Builder[A, Message[A]] = Message.newBuilder
}

Message(1, 2, 3) ++ Message(4, 5, 6)