bounds.scala

def seqPlusHead[S <: Seq[T], T](seq: S): (S, T) = seq -> seq.head

seqPlusHead[List[String], String](List("Hallo", "Welt"))
// -> (List[String], String) = (List(Hallo, Welt),Hallo)

def onlySeqPlusHead[Seq[T], T](seq: Seq[T]) = seqPlusHead[Seq[T], T](seq)
// error: type arguments [Seq[T],T] do not conform to method seqPlusHead's type parameter bounds [S <: Seq[T],T]
//       def onlySeqPlusHead[Seq[T], T](seq: Seq[T]) = seqPlusHead[Seq[T], T](seq)


onlySeqPlusHead[Seq[String], String](Seq(1, 2, 3))