Vector.unapplySeq is refutable

[martijnhoekstra]

reproduction steps

using Scala 2.13.4

scala> Vector(1) match {
  case Vector(_ @ _*) => ???
}

problem

match may not be exhaustive.
It would fail on the following input: Vector1()

The match is totally exhaustive. Unfortunately, scalac doesn't know. Vector.unapplySeq seems to be using name-based pattern matching to return a SeqFactory.UnapplySeqWrapper. But name-based pattern matching can't AFAIK provide irrefutable patterns, so the pattern ends up refutable, and the warning (with the somewhat odd counterexample) is emitted.

[martijnhoekstra]

reported at https://gitter.im/scala/scala?at=5fbbebf6ba69631c7496aaee by @mrdziuban

[mrdziuban]

If/when this is fixed, I would hope that a match statement like this would also be considered exhaustive

Vector("test") match {
  case Vector() => "empty"
  case Vector(_) => "one"
  case Vector(_, _, _*) => "two or more"
}

which seems like a good alternative to using +: like my original question in gitter which can't prove complementary exhaustivity between Vector() and +:, e.g.

Vector("test") match {
  case Vector() => "empty"
  case _ +: Vector() => "one"
  case _ +: _ +: _ => "two or more"
}

[dwijnand]

I wonder if changing UnapplySeqWrapper to have def isEmpty: false.type = false fixes this? Hopefully then two obviously exhaustive cases would be.