Demonstrate generic conversion, where explicit invocations work but implicit invocations don't happen

generic_conversion_example.scala

trait CastOptionToV[V] {
  def cast[T:TypeTag](o:Option[T]):V
}
class CastOptionToLong extends CastOptionToV[Long] {
  def cast[T:TypeTag](o:Option[T]):Long = {
    val tLong = typeOf[Long]
    typeOf[T] match {
      case `tLong` => o.asInstanceOf[Option[Long]].get
      case _ => throw new Exception
    }
  }
}
class CastOptionToString extends CastOptionToV[String] {
  def cast[T:TypeTag](o:Option[T]):String = {
    val tLong = typeOf[Long]
    typeOf[T] match {
      case `tLong` => o.asInstanceOf[Option[Long]].get.toString
      case _ => throw new Exception
    }
  }
}
object CastVals {
  implicit val CO2Long = new CastOptionToLong
  implicit val CO2String = new CastOptionToString
}
import CastVals._

// I can call an explicit caster and it works
def explicitCast[V:CastOptionToV](o:Option[Long]):V = {
    val caster = implicitly[CastOptionToV[V]]
    caster.cast(o)
}

object Cast{
  // This compiles fine, but it doesn't ever get invoked when it's wanted
  implicit def implicitCaster[V:CastOptionToV](o:Option[Long]):V = {
    val caster = implicitly[CastOptionToV[V]]
    caster.cast(o)
  }
}
import Cast._