Variables in detached rulesets not visited by visitor?

I have the following plugin foo.js

module.exports = {
    install({ tree: { Quoted }, visitors }, manager) {
        class Visitor {
            constructor() {
                this.native = new visitors.Visitor(this);

                this.isPreEvalVisitor = true;
                this.isReplacing = true;
            }

            run(root) {
                console.log(
                    root
                        .rules[2]
                        .rules[0]
                        .arguments[0]
                        .value
                        .ruleset
                        .rules[0]
                        .value
                        .value[0]
                        .value[0]
                        .type,
                ); // 'Variable'

                return this.native.visit(root);
            }

            visitVariable(node) {
                console.log('visited');

                return new Quoted(`'`, 'bar', true);
            }
        }

        manager.addVisitor(new Visitor());
    },
};

and the following Less code

@plugin './foo';

.foo(@rules: {}) {
    :root.foo & {
        @rules();
    }
}

.foo {
    .foo({
        --foo: @foo;
    });
}

When trying to compile this code using lessc and/or less-loader, Less fails with error SyntaxError: variable @foo is undefined.

Variables in other contexts (in non-detached rulesets, at top level, in named detached rulests) are visited as expected:

@plugin './foo';

@bar: @foo;

.foo {
    --foo: @foo;
    --bar: @bar;
    @baz();
}

@baz: {
    --baz: @foo;
};

// ↓↓↓↓

.foo {
  --foo: bar;
  --bar: bar;
  --baz: bar;
}

Maybe other node types also affected, but I didn’t test it.

Less version: 3.0.2.

[matthew-dean]

Okay! I figured out that for some reason, MixinCall arguments are not visited. It seems that it’s an array of… objects? instead of nodes? I’ve fixed it, but would like to fix some of the permissive parsing of custom properties to try parsing a value first. Otherwise you’ll get a Variable today, but a Quoted after it’s released, and your visitVariable would stop functioning. I’m just now sure how to make the parser “recover” from a bad path. I’m looking to tweak this code that was merged. - See this comment.

So if someone can help me understand how the Less parser works and addresses that comment, I’ll do a PR for this fix.

[matthew-dean]

Please review the PR - https://github.com/less/less.js/pull/3228