[Twig 4] Introduce ForElseNode

[ruudk]

Currently there is no line number information for {% else %} inside a for loop.

Normally, this wouldn't be such a big deal.

But TwigStan uses this line information to map errors in PHP back to the original Twig source.

Let's say you have the following template:

{% for product in products %}
    <h2>{{ product.name }}</h2>
{% else %}
    <p>No products found</p>
{% endfor %}

And products is of type non-empty-array<Product>, it means that the else will never happen.

This is currently reported as:

Negated boolean expression is always false.
🔖 booleanNot.alwaysFalse
🐘 compiled_index.php:83
🌱 templates/product/index.html.twig:2
🎯 src/Controller/ProductController.php:15

But as you can see, it points to line number 2, instead of line number 3.

In the compiled code, it looks like this:

// line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:1
foreach ($context['_seq'] as $context["_key"] => $context["product"]) {
    // line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:2
    yield "        <h2>";
    // line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:2
    yield \Twig\Extension\CoreExtension::getAttribute($this->env, $this->source, $context
    // line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:2
    yield "</h2>\n    ";
    $context['_iterated'] = \true;
}
if (!$context['_iterated']) {
    // line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:4
    yield "        <p>No products found</p>\n    ";
}

With this change, we will change the compiled code to become:

// line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:1
foreach ($context['_seq'] as $context["_key"] => $context["product"]) {
    // line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:2
    yield "        <h2>";
    // line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:2
    yield \Twig\Extension\CoreExtension::getAttribute($this->env, $this->source, $context
    // line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:2
    yield "</h2>\n    ";
    $context['_iterated'] = \true;
}
// line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:3
if (!$context['_iterated']) {
    // line /Volumes/CS/opensource/twigstan-demo/templates/product/index.html.twig:4
    yield "        <p>No products found</p>\n    ";
}

[stof]

Restricting the type is a BC break. You should introduce the new class in 3.x with a deprecation instead (where passing something else than a ForElseNode or null would trigger a deprecation and wrap the node in a ForElseNode, using the line of that node as fallback line number)