fix(issue:4268) nested pseudo-selector parsing (#4290)

* Fix nested pseudo-selector parsing where selectors are separated by
  one or more commas.

Author: puckowski

packages/less/src/less/parser/parser.js

@@ -5,6 +5,8 @@ import getParserInput from './parser-input';
 import * as utils from '../utils';
 import functionRegistry from '../functions/function-registry';
 import { ContainerSyntaxOptions, MediaSyntaxOptions } from '../tree/atrule-syntax';
+import Selector from '../tree/selector';
+import Anonymous from '../tree/anonymous';
 
 //
 // less.js - parser
@@ -1312,9 +1314,25 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
                 if (!e) {
                     parserInput.save();
                     if (parserInput.$char('(')) {
-                        if ((v = this.selector(false)) && parserInput.$char(')')) {
-                            e = new(tree.Paren)(v);
-                            parserInput.forget();
+                        if ((v = this.selector(false))) {
+                            let selectors = [];
+                            while (parserInput.$char(',')) {
+                                selectors.push(v);
+                                selectors.push(new Anonymous(','));
+                                v = this.selector(false);
+                            }
+                            selectors.push(v);
+                                                        
+                            if (parserInput.$char(')')) {
+                                if (selectors.length > 1) {
+                                    e = new (tree.Paren)(new Selector(selectors));
+                                } else {
+                                    e = new(tree.Paren)(v);
+                                }
+                                parserInput.forget();
+                            } else {
+                                parserInput.restore('Missing closing \')\'');
+                            }
                         } else {
                             parserInput.restore('Missing closing \')\'');
                         }
@@ -1395,7 +1413,9 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
                     } else {
                         if (allExtends) { error('Extend can only be used at the end of selector'); }
                         c = parserInput.currentChar();
-                        if (elements) {
+                        if (Array.isArray(e)){
+                            e.forEach(ele => elements.push(ele));
+                        } if (elements) {
                             elements.push(e);
                         } else {
                             elements = [ e ];

packages/test-data/css/_main/selectors.css

@@ -183,3 +183,9 @@ blank blank blank blank blank blank blank blank blank blank blank blank blank bl
 .first-level .second-level.active2 {
   content: '\2661';
 }
+a:is(.b, :is(.c)) {
+  color: blue;
+}
+a:is(.b, :is(.c), :has(div)) {
+  color: red;
+}

packages/test-data/less/_main/selectors.less

@@ -200,3 +200,11 @@ blank blank blank blank blank blank blank blank blank blank blank blank blank bl
     &.active2:extend(.extend-this) { }
   }
 }
+
+a:is(.b, :is(.c)) {
+  color: blue;
+}
+
+a:is(.b, :is(.c), :has(div)) {
+  color: red;
+}