allowed Twig\NodeVisitor\NodeVisitorInterface::leaveNode() to return "null" instead of "false" (same meaning)

CHANGELOG

@@ -1,5 +1,6 @@
 * 1.40.0 (2019-XX-XX)
 
+ * allowed Twig\NodeVisitor\NodeVisitorInterface::leaveNode() to return "null" instead of "false" (same meaning)
  * added the "apply" tag as a replacement for the "filter" tag
  * allowed Twig\Loader\FilesystemLoader::findTemplate() to return "null" instead of "false" (same meaning)
  * added support for "Twig\Markup" instances in the "in" test

src/NodeTraverser.php

@@ -69,7 +69,7 @@ class NodeTraverser
         $node = $visitor->enterNode($node, $this->env);
 
         foreach ($node as $k => $n) {
-            if (false !== $m = $this->traverseForVisitor($visitor, $n)) {
+            if (false !== ($m = $this->traverseForVisitor($visitor, $n)) && null !== $m) {
                 if ($m !== $n) {
                     $node->setNode($k, $m);
                 }

src/NodeVisitor/AbstractNodeVisitor.php

@@ -17,6 +17,8 @@ use Twig\Node\Node;
 /**
  * Used to make node visitors compatible with Twig 1.x and 2.x.
  *
+ * To be removed in Twig 3.1.
+ *
  * @author Fabien Potencier <fabien@symfony.com>
  */
 abstract class AbstractNodeVisitor implements NodeVisitorInterface
@@ -49,7 +51,7 @@ abstract class AbstractNodeVisitor implements NodeVisitorInterface
     /**
      * Called after child nodes are visited.
      *
-     * @return Node|false The modified node or false if the node must be removed
+     * @return Node|false|null The modified node or null if the node must be removed
      */
     abstract protected function doLeaveNode(Node $node, Environment $env);
 }

src/NodeVisitor/NodeVisitorInterface.php

@@ -30,7 +30,7 @@ interface NodeVisitorInterface
     /**
      * Called after child nodes are visited.
      *
-     * @return \Twig_NodeInterface|false The modified node or false if the node must be removed
+     * @return \Twig_NodeInterface|false|null The modified node or null if the node must be removed
      */
     public function leaveNode(\Twig_NodeInterface $node, Environment $env);