It prints out three WTF's, I expected STRING! STRING! LONG! How to deal with type erasure in this case?

WTF.scala

case class Column[T](s: Option[T] = None)

class Def[C](implicit desired: Manifest[C]) {
  def unapply[X](c: X)(implicit m: Manifest[X]): Option[C] = {
    def sameArgs = desired.typeArguments.zip(m.typeArguments).forall { case (desired, actual) => desired >:> actual }
    if (desired >:> m && sameArgs) Some(c.asInstanceOf[C])
    else None
  }
}

object TestApp {
  val LongTest = new Def[Column[Long]]
  val StringTest = new Def[Column[String]]

  val usersTable = List(
    'email -> Column[String](),
    'name -> Column[String](),
    'id -> Column[Long]())

  def main(args: Array[String]) {
    usersTable.foreach(coldef => {
      val (s: Symbol, f: Column[_]) = coldef
      f match {
        case LongTest(f) => println("LONG!")
        case StringTest(f) => println("STRING!")
        case _ => println("WTF")
      }
    })
  }
}