MyNum.scala

case class MyNum (val d: Double) extends Numeric [MyNum]
{
  def fromInt (n: Int) = MyNum (n)
  def + (rhs: MyNum) = plus(this, rhs)
  def plus (x: MyNum, y: MyNum): MyNum = MyNum (x.d + y.d)
  def minus (x: MyNum, y: MyNum): MyNum = MyNum (x.d - y.d)
  def times (x: MyNum, y: MyNum): MyNum = MyNum (x.d * y.d)
  def negate (x: MyNum): MyNum = MyNum (-x.d)
  def toInt (x: MyNum): Int = x.d.toInt
  def toLong (x: MyNum): Long = x.d.toLong
  def toFloat (x: MyNum): Float = x.d.toFloat
  def toDouble (x: MyNum): Double = x.d.toDouble
  def compare (x: MyNum, y: MyNum) = x.d compare y.d
  override def toString = "%s".format(d)
}

MyNumContainerA.scala

class MyNumContainerA [T : Numeric] (d: T)

MyNumContainerB.scala

class MyNumContainerB [T <% Numeric] (d: T)

MyNumTest.scala

object MyNumTest.scala extends App {
  val a = MyNum (1.0)
  val b = MyNum (3.0)
  val c = a + b // works fine
//val d = new MyNumContainerA (a) // does not compile
  val e = new MyNumContainerB (a)
}