implementation of Equal typeclass using contravariance - difficulties resolved

contravariant.scala

object Test extends App {
  
trait CanEqual[-A]  {
  def equals(a1: A, a2: A): Boolean
}
object CanEqual {
  def apply[A](implicit ev: CanEqual[A]): CanEqual[A] = ev
  def equals[A](f: (A, A) => Boolean): CanEqual[A] = new CanEqual[A] {
    def equals(a1: A, a2: A): Boolean = f(a1, a2)
  }
}
trait CanEqualOps[A] {
  def self: A

//the key difference from Eugene's example is that resolution of the CanEqual typeclass is done on ===, 
//rather than when the first operand is wrapped by CanEqualOps
//this allows us to find a type C that is least-upper-bound of A and B

  final def ===[B <: C, C >: A](other: B)(implicit ev: CanEqual[C]): Boolean = ev.equals(self, other)
}
object ToCanEqualOps {
  implicit def toCanEqualOps[A](v: A) =
    new CanEqualOps[A] {
      def self = v
    }
}

sealed trait TrafficLight
case object Red extends TrafficLight
case object Yellow extends TrafficLight
case object Green extends TrafficLight

implicit val TrafficLightEqual = CanEqual.equals[TrafficLight] {_ == _}

import ToCanEqualOps._

Red === Green
}