:not() selector not working properly with pseudoclasses

[Conduitry]

Describe the bug

As of 5.1.10, selectors like :not(:last-child) are getting erroneously detected as unused.

Reproduction

https://svelte.dev/playground/hello-world?version=5.1.10#H4sIAAAAAAAACm2NQQvCMAyF_0rJaQN17FqH4M3bLt6ch7lGOojNaKNzjP13W3DgwVPyvpf3MoNrHwgaTkjEamRPRmVoekGTwwbuPWEAfZlBpiHdJRD5N3Uchl14IUlitzbgP96xE3QSa6Cy5aF2WBVxNi6p88i_ynpc3UiCTIRpFVtqx5JpaoNsO9uTydWcjEY6JvZaeTT7BJaYK9Zg_C74FtDin7hclw8dQ5rQ7wAAAA==

This does work in the previous version, presumably because we weren't trying to scope the :not() - https://svelte.dev/playground/hello-world?version=5.1.9#H4sIAAAAAAAACm2NQQvCMAyF_0rJaQN17FqH4M3bLt6ch7lGOojNaKNzjP13W3DgwVPyvpf3MoNrHwgaTkjEamRPRmVoekGTwwbuPWEAfZlBpiHdJRD5N3Uchl14IUlitzbgP96xE3QSa6Cy5aF2WBVxNi6p88i_ynpc3UiCTIRpFVtqx5JpaoNsO9uTydWcjEY6JvZaeTT7BJaYK9Zg_C74FtDin7hclw8dQ5rQ7wAAAA==

Logs

Unused CSS selector "h1:not(:last-child)" (css_unused_selector)

System Info

n/a

Severity

annoyance

[kyle-leonhard]

+1, ran into this as well on several selectors of the form: .some-button:not(:disabled):hover. Perhaps related to #13999. fyi @dummdidumm

[Rich-Harris]

It's not just pseudo-classes — see example from #14169 https://svelte.dev/playground/7e9220bafc094e209bb9a53e7e7bb48d?version=5.1.10

[WaltzingPenguin]

Also can be triggered from attributes: https://svelte.dev/playground/7028eeed4643409c860ce6fee36c05ca?version=5.1.10

[dummdidumm]

My mistake was to turn the logic around for all matches, not just the static ones. Static matches are those for static classes, for class: or pseudo classes or attributes they count as a match but shouldn't then be discarded by the negation. I'm not really sure if the current algorithm lets us take this nuance into account when bubbling the result back up, so as a result we possibly have to mark :not selectors as always used

[Rich-Harris]

I don't draw that conclusion — I think we can probably do this (pseudocode):

if (!could_match(selector, node)) {
-  return false;
+  return is_not;
}

return true; // this bit stays the same

In other words 'definitely matches' becomes 'definitely does not match' if you're dealing with a :not while 'definitely does not match' becomes a 'definitely matches', but 'maybe matches' continues to be 'maybe matches'

Tinkering in that direction but I'm not going to get it finished before I need to leave for the evening