I am not sure about the impl, as it e.g. does not work for the regular for loop as we don't have $items[$i] in the scope expressions (don't know whether that is intentional):

/**
 * @param non-empty-list<int> $items
 *
 * @return non-empty-list<int>
 */
function getItemsWithForLoop(array $items): array
{
	for($i = 0; $i < count($items); $i++) {
		$items[$i] = 1;
	}

	assertType('non-empty-list<int>', $items);
	return $items;
}

edit: logged as a new issue so its not forgotten

This pull request has been marked as ready for review.

I have implemented 1) - it works like you suspected.

regarding 2) - what are the expectations with setExistingOffsetValueType? I don't see why it would be more elegant or whether it will cover a situation setOffsetValueType won't. what are the differences between this 2 methods?

setExistingOffsetValueType

Nevermind, probably not a good idea 😊

Thank you!