List[A] => Int without using length

object tony {
  println("Welcome to the Scala worksheet")       //> Welcome to the Scala worksheet

  def foo[A](l:List[A]):Int = {
    l.zipWithIndex.indexWhere { pair => l.indexOf(pair._1) != pair._2 }
  }                                               //> foo: [A](l: List[A])Int

  foo(List(1, 2, 3, 3, 4))                        //> res0: Int = 3

}