scala 2.10 type erasure

gistfile1.scala

scala> class Foo[+A,+B](a: A, b: B)

scala> def hi[A: ClassTag, B: ClassTag](f: Foo[Any,Any]) = f match { case _: Foo[Int,Int] => println("hi"); case _ => println("bye") }
<console>:9: warning: non-variable type argument Int in type pattern Foo[Int,Int] is unchecked since it is eliminated by erasure
       def hi[A: ClassTag,B: ClassTag](f: Foo[Any,Any]) = f match { case _: Foo[Int,Int] => println("hi"); case _ => println("bye") }
                                                                            ^
hi: [A, B](f: Foo[Any,Any])(implicit evidence$1: scala.reflect.ClassTag[A], implicit evidence$2: scala.reflect.ClassTag[B])Unit

scala> hi(new Foo("a","b"))
hi

you are not using A and B here