Less_Parser: Support comma-separated selector lists in pseudo-classes

lib/Less/Parser.php

@@ -2187,9 +2187,28 @@ private function parseElement() {
 			$this->save();
 			if ( $this->matchChar( '(' ) ) {
 				$v = $this->parseSelector();
-				if ( $v && $this->matchChar( ')' ) ) {
-					$e = new Less_Tree_Paren( $v );
-					$this->forget();
+				if ( $v ) {
+					// Support comma-separated selector lists inside parentheses,
+					// for pseudo-classes like :is(), :not(), :where(), :has().
+					// Backported from Less.js 4.2 (less.js#4290).
+					$selectors = [];
+					while ( $this->matchChar( ',' ) ) {
+						$selectors[] = $v;
+						$selectors[] = new Less_Tree_Anonymous( ',' );
+						$v = $this->parseSelector();
+					}
+
+					if ( $v && $this->matchChar( ')' ) ) {
+						if ( $selectors ) {
+							$selectors[] = $v;
+							$e = new Less_Tree_Paren( new Less_Tree_Expression( $selectors, true ) );
+						} else {
+							$e = new Less_Tree_Paren( $v );
+						}
+						$this->forget();
+					} else {
+						$this->restore();
+					}
 				} else {
 					$this->restore();
 				}

test/Fixtures/less.php/css/pseudo-selector-list.css

@@ -0,0 +1,21 @@
+:not(.active) {
+  display: none;
+}
+:not(.a, .b) {
+  display: block;
+}
+.form-small:is(.framework-input, .framework-search, select:not([multiple]):not([size])) {
+  height: 42px;
+}
+a:is(.b, :is(.c), :has(div)) {
+  color: red;
+}
+:where(.x, .y) {
+  margin: 0;
+}
+:has(div > .child, span + .sibling) {
+  border: 1px solid;
+}
+:is([type="text"], .input-field) {
+  padding: 4px;
+}

test/Fixtures/less.php/less/pseudo-selector-list.less

@@ -0,0 +1,22 @@
+// https://github.com/wikimedia/less.php/issues/136
+:not(.active) {
+	display: none;
+}
+:not(.a, .b) {
+	display: block;
+}
+.form-small:is(.framework-input, .framework-search, select:not([multiple]):not([size])) {
+	height: 42px;
+}
+a:is(.b, :is(.c), :has(div)) {
+	color: red;
+}
+:where(.x, .y) {
+	margin: 0;
+}
+:has(div > .child, span + .sibling) {
+	border: 1px solid;
+}
+:is([type="text"], .input-field) {
+	padding: 4px;
+}