Valid syntax not allowed by Stylus when specifying LESS preprocessor

[setyb]

• Browser: Firefox 88.0.1
• Operating System: Win
• Stylus Version: 1.5.17

STR:

1. Create a new userCSS style
2. Paste the code below into Stylus for the new style
3. Try to save the style
4. Stylus will not let you save the style and will issue the following confusing error:

   Missing closing ')'

5. In the code, change /* :is(a, b) */ to /* test */
6. Try to save the style again
7. Stylus will now let you save the style

/* ==UserStyle==
@name         Test Style
@namespace    testX
@version      1.0.0
@author       Dev
@preprocessor less
==/UserStyle== */


@-moz-document domain("example.com") {
	:is([foo], [bar] /* :is(a, b) */, [baz]) {
		color: red !important; }
}

[tophf]

This is a bug in LESS, confirmed by seeing no output in http://lesscss.org/less-preview/
Apparently it doesn't support complex selectors in :is().

If you want to use LESS you'll have to delete the other selectors after the first [foo].
Try reporting to the original repository.

[setyb]

Reported on LESS here: less/less.js#3622