universal equality in the face of subtyping

gistfile1.scala

class Animal(val name: String) {
  override def equals(other: Any): Boolean = other match {
    case that: Animal =>
      (that canEqual this) &&
      (this.name == that.name)
    case _ => false 
  }

  def canEqual(other: Any): Boolean = other.isInstanceOf[Animal]
}

class Cat(name: String, val color: String) extends Animal(name) {
  override def equals(other: Any): Boolean = other match {
    case that: Cat =>
      (that canEqual this) &&
      super.equals(that) && (this.color == that.color)
    case _ => false 
  }

  override def canEqual(other: Any): Boolean = other.isInstanceOf[Cat]
}

val a: Animal = new Animal("Bob")
val b: Animal = new Animal("Bob")
val c: Animal = new Cat("Bob", "Orange")
 
a == b // true
b == a // true
 
a == c // false
c == a // false