gistfile1.txt

object Test extends App {
  trait Term
  class If extends Term

  trait AllowedTransformation[I, O]
  implicit object term2Term extends AllowedTransformation[Term, Term]

  class Foo[T] {
    def filter[U <: T : scala.reflect.ClassTag](f: PartialFunction[U, Boolean]): Foo[U] = {
      println(scala.reflect.classTag[U])
      null
    }
    def update[I <: T : scala.reflect.ClassTag, O <: T](f: PartialFunction[I, O])(implicit x: AllowedTransformation[I, O]): Foo[O] = {
      println(scala.reflect.classTag[I])
      null
    }
  }

  val foo1: Foo[Term] = new Foo[Term]
  // WORKS
  val foo2: Foo[If] = foo1.filter{ case _: If => true }
  // DOESNT WORK
  val foo3: Foo[If] = foo1.update{ case _: If => ??? : If }
}