gistfile1.scala

scala> class Foo(val n: Int)
defined class Foo

scala> class Bar(n: Int) extends Foo(n)
defined class Bar

scala> List[Foo]().sorted
<console>:9: error: No implicit Ordering defined for Foo.
              List[Foo]().sorted
                          ^

scala> implicit val ordFoo = Ordering.by[Foo,Int](_.n)
ordFoo: scala.math.Ordering[Foo] = scala.math.Ordering$$anon$9@15fa2b3e

scala> List[Foo]().sorted
res1: List[Foo] = List()

scala> List[Bar]().sorted
<console>:11: error: No implicit Ordering defined for Bar.
              List[Bar]().sorted
                          ^

scala> implicit def ordFoo[A <: Foo] = Ordering.by[A,Int](_.n)
ordFoo: [A <: Foo]=> scala.math.Ordering[A]

scala> List[Bar]().sorted
res3: List[Bar] = List()