t856.scala:3: error: class ComplexRect needs to be abstract, since:
it has 2 unimplemented members.
/** As seen from class ComplexRect, the missing signatures are as follows.
 *  For convenience, these are usable as stub implementations.
 */
  // Members declared in scala.Equals
  def canEqual(that: Any): Boolean = ???

  // Members declared in scala.Product2
  def _2: Double = ???

class ComplexRect(val _1:Double, _2:Double) extends Complex {
      ^
one error found