pingpongtyperecover.sc

import scala.reflect.ClassTag

trait MyCollection[T]
{
  def ping(c2: MyCollection[T]): Unit
  def pong[C1 <: MyCollection[T] : ClassTag](c1: C1): Unit
  def pung[C1 <: MyCollection[T], C2 <: MyCollection[T]](c1: C1, c2: C2)(implicit C1: ClassTag[C1], C2: ClassTag[C2]): Unit
}

class AListyCollection[T] extends MyCollection[T]
{
  def ping(c2: MyCollection[T]): Unit = c2.pong(this)
  def pong[C1 <: MyCollection[T] : ClassTag](c1: C1): Unit = c1.pung(c1, this)
  def pung[C1 <: MyCollection[T], C2 <: MyCollection[T]](c1: C1, c2: C2)(implicit C1: ClassTag[C1], C2: ClassTag[C2]): Unit =
    println(s"Pingpong at $C1, $C2")
}

class ASettyCollection[T] extends MyCollection[T]
{
  def ping(c2: MyCollection[T]): Unit = c2.pong(this)
  def pong[C1 <: MyCollection[T] : ClassTag](c1: C1): Unit = c1.pung(c1, this)
  def pung[C1 <: MyCollection[T], C2 <: MyCollection[T]](c1: C1, c2: C2)(implicit C1: ClassTag[C1], C2: ClassTag[C2]): Unit =
    println(s"Pingpong at $C1, $C2")
}

val l1 = new AListyCollection[String]
val l2 = new AListyCollection[String]
val s1 = new ASettyCollection[String]
val s2 = new ASettyCollection[String]

l1.ping(l2)
s1.ping(s2)
l1.ping(s2)
s1.ping(l2)

(l1: MyCollection[String]).ping(s2 : MyCollection[String])

Produces:

Pingpong at $file.appending$AListyCollection, $file.appending$AListyCollection
Pingpong at $file.appending$ASettyCollection, $file.appending$ASettyCollection
Pingpong at $file.appending$ASettyCollection, $file.appending$AListyCollection
Pingpong at $file.appending$AListyCollection, $file.appending$ASettyCollection
Pingpong at $file.appending$AListyCollection, $file.appending$ASettyCollection