01-lower-bound.scala

trait Fn[A] {
  def apply[B >: A](t: B): B
}

class Sup
class Sub extends Sup

def test[S >: Sub](a: Sub): Fn[S] =
  new Fn[S] {
    override def apply[B >: S](b: B): B = if (true) a else b
  }

test(new Sub)(new Sub) // res21: scratch.Sub = scratch$Sub@6c994838
//                                       /\
//                                       ||
//                                       \/
test(new Sub)(new Sup) // res22: scratch.Sup = scratch$Sub@450fcee3

02-initial-version.scala

class Sup
class Sub(val element: Int) extends Sup

def test1[B >: Sub](a: Sub, b: B): B = if (a.element < 0) b else a
//                        ||
//                        || difference
//                        ||
//                        \/
def test2[B >: Sub](a: Sub)(b: B): B = if (a.element < 0) b else a

test1(new Sub(1), new Sub(1)) // res17: Sub = Sub@37e5f771
test2(new Sub(1))(new Sub(1)) // res18: Sub = Sub@37900029

test1(new Sub(1), new Sup)    // res19: Sup = Sub@2fa357c9
test2(new Sub(1))(new Sup)    // WTF?!
// <console>:11: error: type mismatch;
//  found   : Sup
//  required: Sub
//               test2(new Sub(1))(new Sup)

03-function-like.scala

  trait Fn[A,B] {
    // AA is contravariant; BB is covariant
    // Similar to Function1[-AA, +BB]
    def apply[AA >: A, BB <: B](t: AA): BB
  }

  class Sup
  class Sub extends Sup

  def test[S >: Sub](a: Sub): Fn[S,S] =
    new Fn[S,S] {
      override def apply[AA >: S, BB <: S](b: AA): BB = if (true) a else b
    }