Extending inner of :not can lead to invalid css

[mgreter]

Extracted from #2051

:not(.thing) {
    color: red;
}
:not(.bar) {
    @extend .thing;
    background: blue;
}

Produces:

:not(.thing):not(:not(.bar)) {
  color: red; }
:not(.bar) {
  background: blue; }

But should be:

:not(.thing) {
  color: red; }
:not(.bar) {
  background: blue; }

I wonder if ruby sass actually just removes the invalidly nested :not selector.
Otherwise from what I know about extend, the result in theory doesn't look incorrect!?