Bound a type parameter to be strictly a subtype.

StrictSubtype.scala

object Test extends App {

  // From https://stackoverflow.com/a/6944070

  trait =!=[A, B]

  implicit def neq[A, B] : A =!= B = null

  // This pair excludes the A =:= B case
  implicit def neqAmbig1[A] : A =!= A = null
  implicit def neqAmbig2[A] : A =!= A = null

  type ¬[T] = T => Nothing
  implicit def neg[T, U](t : T)(implicit ev : T =!= U) : ¬[U] = null

  // An example:

  class Foo(val string: String)
  class Bar(val extra: String) extends Foo("bar")


  /** Require an argument which is strictly a subtype of [[Foo]]. */
  def wut[A <: Foo](t : A)(implicit ev: A =!= Foo): Foo = t

  val bar = new Bar("Hello")
  val foo = new Foo("nope")

  println(bar)
  println(foo)

  println(wut(bar))

//Error: Could not find implicit value for parameter ev: Test.Foo =!= Test.Foo
//println(wut(foo))

}