AnyRefMap iterates its way to ((null, null))

[scabug]

You can't cache the result of hasNext on an iterator showing a live view of a mutable collection.

def f() = {
  val xs = scala.collection.mutable.AnyRefMap[String, Int]("a" -> 1)
  val it = xs.iterator
  it.hasNext
  xs.clear()

  if (it.hasNext) Some(it.next)
  else None
}
// scala> f
// res0: Option[(String, Int)] = Some((null,null))

The funny thing is that the AnyRefMap iterator makes copies of _keys and _values in what might be an attempt to avoid this, but then it uses the originals instead of the copies (this is something -Xlint would tell you if you used it) and it wouldn't avoid this even if iterator referenced kz and vz instead of _keys and _values. Maybe something else is in mind there.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8154?orig=1
Reporter: @paulp
See #8153

[scabug]

@Ichoran said:
Don't know what I was thinking there. I refactored that code way too many times. Thanks for catching that.

[scabug]

@Ichoran said:
scala/scala#3371

But it looks like fixes I've made in the meantime will require me to manually merge.