diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-includes/class-wp-walker.php
--- a/wp/wp-includes/class-wp-walker.php	Tue Jun 09 11:14:17 2015 +0000
+++ b/wp/wp-includes/class-wp-walker.php	Mon Oct 14 17:39:30 2019 +0200
@@ -16,7 +16,6 @@
 	 * What the class handles.
 	 *
 	 * @since 2.1.0
-	 * @access public
 	 * @var string
 	 */
 	public $tree_type;
@@ -56,7 +55,7 @@
 	 * @since 2.1.0
 	 * @abstract
 	 *
-	 * @param string $output Passed by reference. Used to append additional content.
+	 * @param string $output Used to append additional content (passed by reference).
 	 * @param int    $depth  Depth of the item.
 	 * @param array  $args   An array of additional arguments.
 	 */
@@ -71,7 +70,7 @@
 	 * @since 2.1.0
 	 * @abstract
 	 *
-	 * @param string $output Passed by reference. Used to append additional content.
+	 * @param string $output Used to append additional content (passed by reference).
 	 * @param int    $depth  Depth of the item.
 	 * @param array  $args   An array of additional arguments.
 	 */
@@ -86,7 +85,7 @@
 	 * @since 2.1.0
 	 * @abstract
 	 *
-	 * @param string $output            Passed by reference. Used to append additional content.
+	 * @param string $output            Used to append additional content (passed by reference).
 	 * @param object $object            The data object.
 	 * @param int    $depth             Depth of the item.
 	 * @param array  $args              An array of additional arguments.
@@ -102,7 +101,7 @@
 	 * @since 2.1.0
 	 * @abstract
 	 *
-	 * @param string $output Passed by reference. Used to append additional content.
+	 * @param string $output Used to append additional content (passed by reference).
 	 * @param object $object The data object.
 	 * @param int    $depth  Depth of the item.
 	 * @param array  $args   An array of additional arguments.
@@ -122,17 +121,16 @@
 	 * @since 2.5.0
 	 *
 	 * @param object $element           Data object.
-	 * @param array  $children_elements List of elements to continue traversing.
+	 * @param array  $children_elements List of elements to continue traversing (passed by reference).
 	 * @param int    $max_depth         Max depth to traverse.
 	 * @param int    $depth             Depth of current element.
 	 * @param array  $args              An array of arguments.
-	 * @param string $output            Passed by reference. Used to append additional content.
-	 * @return null Null on failure with no changes to parameters.
+	 * @param string $output            Used to append additional content (passed by reference).
 	 */
 	public function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) {
-
-		if ( !$element )
+		if ( ! $element ) {
 			return;
+		}
 
 		$id_field = $this->db_fields['id'];
 		$id       = $element->$id_field;
@@ -140,7 +138,7 @@
 		//display this element
 		$this->has_children = ! empty( $children_elements[ $id ] );
 		if ( isset( $args[0] ) && is_array( $args[0] ) ) {
-			$args[0]['has_children'] = $this->has_children; // Backwards compatibility.
+			$args[0]['has_children'] = $this->has_children; // Back-compat.
 		}
 
 		$cb_args = array_merge( array(&$output, $element, $depth), $args);
@@ -149,7 +147,7 @@
 		// descend only when the depth is right and there are childrens for this element
 		if ( ($max_depth == 0 || $max_depth > $depth+1 ) && isset( $children_elements[$id]) ) {
 
-			foreach( $children_elements[ $id ] as $child ){
+			foreach ( $children_elements[ $id ] as $child ){
 
 				if ( !isset($newlevel) ) {
 					$newlevel = true;
@@ -188,16 +186,14 @@
 	 * @param int   $max_depth The maximum hierarchical depth.
 	 * @return string The hierarchical item output.
 	 */
-	public function walk( $elements, $max_depth) {
-
+	public function walk( $elements, $max_depth ) {
 		$args = array_slice(func_get_args(), 2);
 		$output = '';
 
-		if ($max_depth < -1) //invalid parameter
+		//invalid parameter or nothing to walk
+		if ( $max_depth < -1 || empty( $elements ) ) {
 			return $output;
-
-		if (empty($elements)) //nothing to walk
-			return $output;
+		}
 
 		$parent_field = $this->db_fields['parent'];
 
@@ -218,7 +214,7 @@
 		$top_level_elements = array();
 		$children_elements  = array();
 		foreach ( $elements as $e) {
-			if ( 0 == $e->$parent_field )
+			if ( empty( $e->$parent_field ) )
 				$top_level_elements[] = $e;
 			else
 				$children_elements[ $e->$parent_field ][] = $e;
@@ -253,7 +249,7 @@
 		if ( ( $max_depth == 0 ) && count( $children_elements ) > 0 ) {
 			$empty_array = array();
 			foreach ( $children_elements as $orphans )
-				foreach( $orphans as $op )
+				foreach ( $orphans as $op )
 					$this->display_element( $op, $empty_array, 1, 0, $args, $output );
 		 }
 
@@ -272,15 +268,16 @@
 	 *
  	 * @since 2.7.0
 	 *
- 	 * @param int $max_depth The maximum hierarchical depth.
- 	 * @param int $page_num  The specific page number, beginning with 1.
- 	 * @return string XHTML of the specified page of elements
- 	 */
+	 * @param array $elements
+	 * @param int   $max_depth The maximum hierarchical depth.
+	 * @param int   $page_num The specific page number, beginning with 1.
+	 * @param int   $per_page
+	 * @return string XHTML of the specified page of elements
+	 */
 	public function paged_walk( $elements, $max_depth, $page_num, $per_page ) {
-
-		/* sanity check */
-		if ( empty($elements) || $max_depth < -1 )
+		if ( empty( $elements ) || $max_depth < -1 ) {
 			return '';
+		}
 
 		$args = array_slice( func_get_args(), 4 );
 		$output = '';
@@ -376,15 +373,22 @@
 		if ( $end >= $total_top && count( $children_elements ) > 0 ) {
 			$empty_array = array();
 			foreach ( $children_elements as $orphans )
-				foreach( $orphans as $op )
+				foreach ( $orphans as $op )
 					$this->display_element( $op, $empty_array, 1, 0, $args, $output );
 		}
 
 		return $output;
 	}
 
+	/**
+	 * Calculates the total number of root elements.
+	 *
+	 * @since 2.7.0
+	 *
+	 * @param array $elements Elements to list.
+	 * @return int Number of root elements.
+	 */
 	public function get_number_of_root_elements( $elements ){
-
 		$num = 0;
 		$parent_field = $this->db_fields['parent'];
 
@@ -395,11 +399,18 @@
 		return $num;
 	}
 
-	// Unset all the children for a given top level element.
+	/**
+	 * Unset all the children for a given top level element.
+	 *
+	 * @since 2.7.0
+	 *
+	 * @param object $e
+	 * @param array $children_elements
+	 */
 	public function unset_children( $e, &$children_elements ){
-
-		if ( !$e || !$children_elements )
+		if ( ! $e || ! $children_elements ) {
 			return;
+		}
 
 		$id_field = $this->db_fields['id'];
 		$id = $e->$id_field;
@@ -408,9 +419,7 @@
 			foreach ( (array) $children_elements[$id] as $child )
 				$this->unset_children( $child, $children_elements );
 
-		if ( isset($children_elements[$id]) )
-			unset( $children_elements[$id] );
-
+		unset( $children_elements[ $id ] );
 	}
 
 } // Walker