implicits.scala

object Ok {
  trait Foo[A]
  case class Bar(x: Int)
  object Bar {
    implicit class BarOps(bar: Foo[Bar]) {
      val ok = "ok"
    }
  }

  def usage(bar: Foo[Bar]) = bar.ok // Ok, the implicit conversion is found in the Bar companion object
}

// Same with Foo[A] being an abstract type member
trait Ko {
  type Foo[A]
  case class Bar(x: Int)
  object Bar {
    implicit class BarOps[A](bar: Foo[Bar]) {
      val ko = "ko"
    }
  }

  def usage(bar: Foo[Bar]) = bar.ko // Does not compile: “value ko is not a member of Ko.this.Foo[Ko.this.Bar]”
}