Complementing irrefutable unapplySeq aren't recognized as exhaustive

[martijnhoekstra]

reproduction steps

using any version of Scala

object SeqExtractor {
  def unapplySeq(x: Int): Some[List[Int]] = Some(List(1))
}

1 match {
 case SeqExtractor() => "zero"
 case SeqExtractor(_) => "one"
 case SeqExtractor(_, _, _*) => "two or more"
}

problem

SeqExtractor is irrefutable, and the three cases together are exhaustive, but we fail to recognize that.

note

Matching on List can work around that by matching on Cons or Nil exhaustively with

List(1) match {
case Nil => 0 // or List() due to rewrite
case head :: Nil => 1
case head :: second :: tail => 2
}

Spun out of #12240