Scala Language Specification v. 2.9 (24 may 2011), Section 8.1.9 "Pattern Sequences" reads:

"A pattern sequence p 1 , . . . , p n appears in two contexts. First, in a constructor pattern c(q 1 , . . . , q m , p 1 , . . . , p n ), where c is a case class which has m + 1 primary constructor parameters, ending in a repeated parameter (§4.6.2) of type S∗. Second,
in an extractor pattern x(p 1 , . . . , p n ) if the extractor object x has an unapplySeq
method with a result type conforming to Seq[S], but does not have an unapply
method that matches p 1 , . . . , p n ."

The second case seems to be incorrect and incomplete:

• All unapplySeq examples I've found actually have return types conforming to Option[Seq[S]], not Seq[S]
• Some examples (e.g. scala.xml.Elem) have return types confirming to Option[(,...,Seq[S])]