no-common-type-error.scala

import java.util.ArrayList

object Scratch

trait Empty[+E[_]] {
  def empty[A]: E[A]
}

object Emptys {
  def <∅>[E[_], A](implicit e: Empty[E]): E[A] = e.empty
}

object Empty {
  implicit def JavaArrayListEmpty: Empty[ArrayList] = new Empty[ArrayList] {
    def empty[A] = new ArrayList[A]
  }
}

trait Functor[F[_]] {
  def fmap[A, B](r: F[A], f: A => B): F[B]
}

object Functor {
  implicit def JavaCollectionFunctor[S[X] <: java.util.Collection[X]: Empty]: Functor[S] = new Functor[S] {
    def fmap[A, B](r: S[A], f: A => B) = {
      import Emptys._
      val a: S[B] = <∅>[S, B]
      val i = r.iterator
      while (i.hasNext)
        a.add(f(i.next))
      a
    }
  }
}

implicitly[Functor[ArrayList]]