It took me many hours to narrow down a problem in some code to this reproducible error, which seems to me like a bug in AVM2. Can anyone shed light on why this is occurring or how to fix it?

When the value at index 1 is deleted and a value is subsequently set at index 0, the non-existent (undefined) value at index 1 will now show up in a foreach loop. I have only been able to produce this outcome with index 1 and 0 (not any other n and n-1).

Run this code:

package 
{
import flash.display.Sprite;
public class Main extends Sprite 
{
    public function Main():void 
    {
        var bar : Array = new Array(6);
        out(bar);

        //proper behavior
        trace("bar[1] = 1", bar[1] = 1);
        out(bar);

        //proper behavior
        trace("delete bar[1]", delete bar[1]);
        out(bar);

        //proper behavior
        trace("bar[4] = 4", bar[4] = 4);
        out(bar);

        //for each loop will now iterate over the undefined position at index 1
        trace("bar[0] = 0", bar[0] = 0);
        out(bar);

        trace("bar[3] = 3", bar[3] = 3);
        out(bar);
    }

    private function out(bar:Array):void
    {
        trace(bar);
        for each(var i : * in bar)
        {
            trace(i);
        }
    }
}

}

It will give this output:

,,,,,
bar[1] = 1 1
,1,,,,
1
delete bar[1] true
,,,,,
bar[4] = 4 4
,,,,4,
4
bar[0] = 0 0
0,,,,4,
0
undefined
4
bar[3] = 3 3
0,,,3,4,
0
undefined
4
3