Usage of existential type in Scala

Main.scala

object Main {
  class MyArray[A:Manifest](size: Int) {
    private[this] val elements = new Array[A](size)

    def get(index: Int): A = {
      elements(index)
    }

    def set(index: Int, element: A): Unit = {
      elements(index) = element
    }

    def indexOf[B >: A](target: B): Int = {
      for((e, i) <- elements.zipWithIndex) {
        if(e == target) return i
      }
      -1
    }
  }
  def main(args: Array[String]): Unit = {
    val a1: MyArray[String] = new MyArray(2)
    a1.set(0, "A")
    a1.set(1, "B")
    val a2: MyArray[_ <: Any] = a1
    println(a2.indexOf("B")) // 1
  }
}