scala> Seq(1,2).view ++ Seq(3,4)
res0: scala.collection.SeqView[Int,Seq[_]] = SeqViewA(...)

scala> Seq(1,2).view ++: Seq(3,4)
res1: Seq[Int] = SeqViewA(...)

scala> Seq(1,2).view ++: List(3,4)
java.lang.ClassCastException: scala.collection.SeqViewLike$$anon$2 cannot be cast to scala.collection.immutable.List
  ... 33 elided

scala> Seq(1,2).view ++: Vector(3,4)
java.lang.ClassCastException: scala.collection.SeqViewLike$$anon$2 cannot be cast to scala.collection.immutable.Vector
  ... 33 elided

scala> List(1,2) ++ Seq(1,2).view
res4: List[Int] = List(1, 2, 1, 2)

scala> List(1,2) ++: Seq(1,2).view
java.lang.UnsupportedOperationException: TraversableView.Builder.result
  at scala.collection.TraversableView$NoBuilder.result(TraversableView.scala:27)
  at scala.collection.TraversableView$NoBuilder.result(TraversableView.scala:24)
  at scala.collection.TraversableLike$class.$plus$plus(TraversableLike.scala:159)
  at scala.collection.immutable.List.$plus$plus(List.scala:196)
  at scala.collection.TraversableLike$class.$plus$plus$colon(TraversableLike.scala:236)
  at scala.collection.SeqLike$$anon$2.$plus$plus$colon(SeqLike.scala:646)
  ... 33 elided