When rendering the following template, I get the error A block definition cannot be nested under non-capturing nodes:

{% extends "layout.twig" %}

{% if false %}
    {% block content %}FOO{% endblock %}
{% endif %}

(note that there is no need for the layout.twig template to exist for the error to be thrown)

But when rendering the following one, I don't get any error:

{% if false %}
    {% block content %}FOO{% endblock %}
{% endif %}

In both cases, there is a block definition nested under a non-capturing node (if), so why is the first template failing and why is the second one not failing?