fix(mixing): resolves issue #4234

[puckowski]

What:

Fix mixin arity issue for mixins that provide a default value for arguments early in the list that caused wrong number of arguments error. Resolves #4234

Why:

This Less:

.mixin(@a) {
  value: @a;
}
.mixin(@a: 1, @b) {
  value: @a + @b;
}

.box {
  .mixin(3);
}

results in wrong number of arguments for .mixin (1 for 2) but should be valid.

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

Summary by CodeRabbit

• Bug Fixes

  • Improved LESS mixin argument matching for positional and named arguments.
  • Added more accurate validation for required, optional, patterned, and variadic parameters.
  • Corrected handling of overloaded mixins with different argument patterns.

• Tests

  • Added coverage for mixins called with positional and named arguments.
  • Added expected CSS output for the new arity-matching scenarios.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b5cd4b2d-7572-409c-b1ce-4f0453c37462

📥 Commits

Reviewing files that changed from the base of the PR and between 937300c and 944caa6.

📒 Files selected for processing (3)

• packages/less/lib/less/tree/mixin-definition.js
• packages/test-data/tests-unit/mixins-named-args/mixins-named-args.css
• packages/test-data/tests-unit/mixins-named-args/mixins-named-args.less

---

📝 Walkthrough

Walkthrough

Changes

The mixin matcher now assigns named and positional arguments by parameter slot, validates arity and required values, and evaluates pattern-like parameters across the full mixin arity. Tests add overloaded mixins with positional and named calls and expected CSS output.

Mixin argument matching

Layer / File(s)	Summary
Argument assignment and pattern matching packages/less/lib/less/tree/mixin-definition.js	Definition.matchArgs separates positional arguments, assigns named arguments to unfilled parameters, validates missing or extra arguments, and compares evaluated pattern values.
Positional and named arity coverage packages/test-data/tests-unit/mixins-named-args/mixins-named-args.less, packages/test-data/tests-unit/mixins-named-args/mixins-named-args.css	Adds overloaded mixin definitions and verifies positional and named calls produce values 3 and 4.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise and accurately points to the mixin arity fix tied to issue #4234.
Linked Issues check	✅ Passed	The change addresses the reported arity-matching bug and adds tests for positional and named mixin calls.
Out of Scope Changes check	✅ Passed	The changes stay focused on mixin arity matching and related test fixtures, with no obvious unrelated edits.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[greptile-apps]

Greptile Summary

This PR fixes argument matching for Less mixins with required parameters after defaulted parameters. The main changes are:

• Maps named arguments to their parameter slots before positional arguments.
• Rejects unknown and excess arguments after slot assignment.
• Adds positional and named regression cases for the corrected arity behavior.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• Argument matching now follows the same named-first binding model used during parameter evaluation.
• Required, defaulted, variadic, and literal-pattern parameter paths retain explicit checks.

_{Reviews (1): Last reviewed commit: "fix(mixing): resolves issue #4234" | Re-trigger Greptile}