Dotty: how to filter a tuple

Question
1

I looked at the test:

type P[x] <: Boolean = x match {
  case 3 => false
  case _ => true
}
type RejectAll[x] = false

def Test =
  summon[Tuple.Filter[(1, 2, 3, 4), P] =:= (1, 2, 4)]
  summon[Tuple.Filter[(1, 2, 3, 4), RejectAll] =:= EmptyTuple]
  summon[Tuple.Filter[EmptyTuple, P] =:= EmptyTuple]

So type matching allows us to get the correct type, however that does not provide an implementation to filter a tuple. So how do we filter tuples?

TIA.