code: protected function checkLoopUsageCondition(Twig_TokenStream $stream, Twig_NodeInterface $node)
      {
          if ($node instanceof Twig_Node_Expression_GetAttr && $node->getNode('node') instanceof Twig_Node_Expression_Name && 'loop' == $node->getNode('node')->getAttribute('name')) {
              throw new Twig_Error_Syntax('The "loop" variable cannot be used in a looping condition.', $node->getTemplateLine(), $stream->getSourceContext());
          }
  
          foreach ($node as $n) {
              if (!$n) {
                  continue;
              }
  
              $this->checkLoopUsageCondition($stream, $n);
          }
      }
docs: the loop variable cannot be used in the condition