Strange pattern matching behaviour with AnyRef

Question

def test1(a: Any) = a match {
  case x: AnyRef => \"AnyRef\"
  case _ => \"None of the above\"
}

def test2(a: Any) = a match {
  case x: Double if x > 2 =>          


        

Answer 1

This is because Any is actually just an Object. Having Double there is a convenient fiction--it's actually java.lang.Double which is autounboxed for you in the match statement. Unfortunately, there is no way for Scala to tell if it finds a java.lang.Double if it is supposed to be interpreted as a Double or as a java.lang.Double--in the latter case, the AnyRef should catch it. So it does. But if you specifically ask for a Double, it knows it is supposed to unbox, and then the AnyRef case need not be checked. (And, in fact, if you intended it to be a java.lang.Double, it will unbox that too--it can't tell the difference.)

Whether this is ideal behavior is debatable, but it is logical.