Incorrect behavior with `take` on empty vector iterator

[adampauls]

reproduction steps

using Scala 2.13.7

object Test {
  def main(args: Array[String]): Unit = {
    val xx: Vector[Int] = Vector.empty[Int]
    val rev = xx.reverseIterator
    assert(rev.isEmpty)
    val afterTake = rev.take(1)
    assert(afterTake.isEmpty) // fails but shouldn't
  }
}

problem

The second assert failes and shouldn't. Attempting to use afterTake will crash with an IndexOutOfBoundException.

[SethTisue]

@scala/collections ?

[SethTisue]

minimized:

scala 2.13.7> Vector.empty[Int].reverseIterator.take(1).toList
java.lang.IndexOutOfBoundsException: 0 is out of bounds (empty vector)
  at scala.collection.immutable.Vector0$.ioob(Vector.scala:371)

[som-snytt]

It is empty. res4 is the lie.

Welcome to Scala 2.13.7-20211029-123143-2d36e71 (OpenJDK 64-Bit Server VM, Java 17).
Type in expressions for evaluation. Or try :help.

scala> Vector.empty[Int]
val res0: scala.collection.immutable.Vector[Int] = Vector()

scala> .reverseIterator
val res1: Iterator[Int] = <iterator>

scala> .isEmpty
val res2: Boolean = true

scala> res1.take(1)
val res3: Iterator[Int] = <iterator>

scala> .isEmpty
val res4: Boolean = false

scala> res3.next()
java.lang.IndexOutOfBoundsException: 0 is out of bounds (empty vector)
  at scala.collection.immutable.Vector0$.ioob(Vector.scala:371)
  at scala.collection.immutable.Vector0$.apply(Vector.scala:336)
  at scala.collection.immutable.Vector0$.apply(Vector.scala:334)
  at scala.collection.SeqView$Id.apply(SeqView.scala:50)
  at scala.collection.IndexedSeqView$IndexedSeqViewReverseIterator.next(IndexedSeqView.scala:93)
  ... 34 elided

[som-snytt]

I remember several years ago that tweaking slice iterators is so tricky.

I didn't try to understand what startCutoff and nextStartCutoff represent.

Are they releasing 2.13.8 soon? would be nice to sneak this one in.

It's an existing bug in the view, but the regression is that IndexedSeq#reverseIterator now uses view.

Welcome to Scala 2.13.6 (OpenJDK 64-Bit Server VM, Java 17).
Type in expressions for evaluation. Or try :help.

scala> Vector.empty[Int].view
val res0: scala.collection.IndexedSeqView[Int] = IndexedSeqView(<not computed>)

scala> .reverseIterator
val res1: Iterator[Int] = <iterator>

scala> .take(1)
val res2: Iterator[Int] = <iterator>

scala> .isEmpty
val res3: Boolean = false

[SethTisue]

Ah, I hadn't realized this was a 2.13.7 specific regression. Presumably from scala/scala#9258.

This does put some pressure on us not to wait as long this time before building 2.13.8. (There was a five and a half month gap after 2.13.6.) I'm not sure how much pressure.

[som-snytt]

Yes, that's the one. I was going to joke that it really does fail faster. Just a little iterator humo(u)r.

[NthPortal]

yup, I think you're right about the cause Seth. technically not my bug, but my fault it now happens without calling view first