Transform this TypedSource into another by mapping after.
Transform this TypedSource into another by mapping after. We don't call this map because of conflicts with Mappable, unfortunately
Because TupleConverter cannot be covariant, we need to jump through this hoop.
Because TupleConverter cannot be covariant, we need to jump through this hoop. A typical implementation might be: (implicit conv: TupleConverter[T]) and then:
override def converter[U >: T] = TupleConverter.asSuperConverter[T, U](conv)