How can I solve the Diverging implicit expansion for type

Question

I want to make my case class Event[K, V] be ordered by key K always. But I need to be able to compare Events of different values V . How can I solve this diverging implicit expansion?

import scala.math.Ordering

object Event {
  case class Event[K, V](key: K, value: V)
    (implicit o: Ordering[K]) extends Ordered[Event[K, _]] {
      override def compare(that: Event[K, _]): Int = o.compare(key, that.key)
    }
}

object Main extends App {
  // mimicking a librarys function 
  def lala[O](e: O)(implicit ordering: Ordering[O]) = ???

  val b = Event.Event("a", 12) <= Event.Event("a", 11.99)    
  lala(Event.Event("a", 12))
}

The call to lala does not compile because of this diverging implicit expansion:

diverging implicit expansion for type   
scala.math.Ordering[Event.Event[String,Int]] starting with method $conforms 
in object Predef lala(Event.Event("a", 12))

Answer 1

If your library method expects an Ordering instance for your type, you should provide that indead of extended Ordered , which is totally underalted from a compiler point of view. Try the following instead:

case class Event[K, V](key: K, value: V)

object Event {
  implicit def eventOrdering[K, V](implicit o: Ordering[K]) =
    new Ordering[Event[K, V]] {
      // ...
    }
}