Foldable.iterateRight is not sound

[johnynek]

similar to #1716 I think Foldable.iterateRight will break if Eval.always are always used.

I think the fix is that we must accept an Iterable[A] rather than Iterator[A] and call .iterator internally. Basically add a Eval.always(iter.iterator).flatMap { ... } and I think it should be safe.

Of course, we need to demonstrate the problem with a failing test first.

[LukaJCB]

Found a test that doesn't work:

forAll { (fa: List[Int]) =>
  val eval = Foldable.iterateRight(fa, Eval.later(0)) { (a, eb) =>
    Eval.always(a + eb.value)
  }
  eval.value should === (fa.sum)
  //Repeat here so the result is evaluated again
  eval.value should === (fa.sum)
}

[LukaJCB]

Fixed by #1980