Add nonfinal subclass selector test

Author: sirreal

src/wp-includes/html-api/class-wp-css-complex-selector-list.php

@@ -134,9 +134,13 @@ final protected static function parse_complex_selector( string $input, int &$off
 				return null;
 			}
 
-			// `div > .className` is valid, but `.className > div` is not.
+			/*
+			 * Subclass selectors in non-final position is not supported:
+			 *   - `div > .className` is valid
+			 *   - `.className > div` is not
+			 */
 			if ( $has_preceding_subclass_selector ) {
-				throw new Exception( 'Unsupported non-final subclass selector.' );
+				return null;
 			}
 			$has_preceding_subclass_selector = null !== $selector->subclass_selectors;
 

tests/phpunit/tests/html-api/wpCssComplexSelectorList.php

@@ -59,6 +59,16 @@ public function test_parse_invalid_complex_selector() {
 		$this->assertNull( $result );
 	}
 
+	/**
+	 * @ticket TBD
+	 */
+	public function test_parse_invalid_complex_selector_nonfinal_subclass() {
+		$input  = 'el.foo#bar[baz=quux] > final, rest';
+		$offset = 0;
+		$result = $this->test_class::test_parse_complex_selector( $input, $offset );
+		$this->assertNull( $result );
+	}
+
 	/**
 	 * @ticket TBD
 	 */