What does Equivalence of instances mean?

Question
11

Something like this compiles:

// Start writing your ScalaFiddle code here
abstract class Bdd (val ident:Int) {}

case class BddNode(label:Int, positive:Bdd, negative:Bdd) extends Bdd(Bdd.nextIdent()) {
  override def equals(that:Any):Boolean = {
    this eq that.asInstanceOf[AnyRef]
  }
  
  override def hashCode(): Int =
    System.identityHashCode(this)
}

object Bdd {
  var currentIdent = 2

  def nextIdent(): Int = {
    currentIdent += 1
    currentIdent - 1
  }
}

and gives following result:

val a = BddNode(5, null, null)
val b = BddNode(5, null, null)
println(a == b) // prints false

PS:
I’m not sure if overridding hashCode is needed. In fact fixed hashCode compiles with contract, but performance is impacted.