lib/Twig/lib/Twig/TokenParser/For.php
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);
        }
    }
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);
        }
    }
the loop variable cannot be used in the condition