code: protected function checkLoopUsageBody(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')) {
              $attribute = $node->getNode('attribute');
              if ($attribute instanceof Twig_Node_Expression_Constant && in_array($attribute->getAttribute('value'), array('length', 'revindex0', 'revindex', 'last'))) {
                  throw new Twig_Error_Syntax(sprintf('The "loop.%s" variable is not defined when looping with a condition.', $attribute->getAttribute('value')), $node->getTemplateLine(), $stream->getSourceContext());
              }
          }
  
          // should check for parent.loop.XXX usage
          if ($node instanceof Twig_Node_For) {
              return;
          }
  
          foreach ($node as $n) {
              if (!$n) {
                  continue;
              }
  
              $this->checkLoopUsageBody($stream, $n);
          }
      }
docs: it does not catch all problems (for instance when a for is included into another or when the variable is used in an include)