I noticed strange behaviour with `ArrayDeque<E&...

# stdlib

[Szymon Jeziorski]

I noticed strange behaviour with ArrayDeque<E> and its implementation of MutableList<E>.removeAll(elements: Collection<E>), it doesn't work when elements are view created by subList. Method's invocation doesn't produce any kind of RuntimeException, but no elements are actually removed.

example:

val arrayDeque = ArrayDeque((1..10).toList())
val subList = arrayDeque.subList(arrayDeque.size - 3, arrayDeque.size)
arrayDeque.removeAll(subListł)
println(arrayDeque) // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

when passed new list object as an argument for example by calling slice, it all works fine:

val arrayDeque = ArrayDeque((1..10).toList())
val subList = arrayDeque.slice(arrayDeque.size - 3..arrayDeque.lastIndex)
arrayDeque.removeAll(subList)
println(arrayDeque) // [1, 2, 3, 4, 5, 6, 7]

Using different kind of MutableList also works just fine:

val mutableList = (1..10).toMutableList()
val subList = mutableList.subList(mutableList.size - 3, mutableList.size)
mutableList.removeAll(subList)
mutableList // [1, 2, 3, 4, 5, 6, 7]

Also, invoking .subList(...).clear(...) on ArrayDeque results in the same issue, no elements are being removed
Should this be reported as a bug?

[Szymon Jeziorski]

In the end I've created an issue for it just in case anyone wanted to follow:
https://youtrack.jetbrains.com/issue/KT-55285

[Klitos Kyriacou]

This is working as per the documentation of `subList`: "Structural changes in the base list make the behavior of the view undefined."

[Szymon Jeziorski]

But the issue here is not with the behavior of the view, but with the behavior of base list, in my example I don't necessarily care about how the view is affected after removeAll call.

As per documentation of `removeAll`: "Removes all of this collection's elements that are also contained in the specified collection"

public override fun removeAll(elements: Collection<E>): Boolean = filterInPlace { !elements.contains(it) }

from ArrayDeque would also suggest logic boiling down to iterating through view elements and removing all elements from base that are also contained in view. Cannot see how elements being view should change its behavior

[Klitos Kyriacou]

Since subList returns a view of the original list, and you are modifying the original list, the behaviour of the view is undefined. Since you are modifying the original list based on a view that is undefined, the result of the operation is therefore also undefined.

[Szymon Jeziorski]

I expected ArrayDeque to act similarly to ArrayList as in provided example, but you're right that this expectation might have been incorrect. Will therefore avoid using views for any mutations of the original collections in the future, thanks!