Problem is that following code is legal (this is not specified in section 6.7):

trait SumExample {
  def sum(x: Int*) = x.sum
  val fun = sum _
  fun(1, 2, 3)
}

Also it's strange that there is no presentation in code for type Int* => Int.
So Scala should have syntax like (x: Int*) => x.sum for function literals. And this should be syntax sugar for type like FunctionSeq1[Int, Int] {def apply(x: Int*): Int = x.sum}.
Or this should be removed from language. For example sum _ can have type Seq[Int] => Int.