{# main.twig #}
{% set test = false %}

<h1>Hello</h1>

{% block dialog %}
    dialog base
{% endblock %}

{# test.twig #}

{% extends 'main.twig' %}

{% if false and test %}

    {% block dialog %}

        dialog override

        {% if test == true %}
            test is true
        {% else %}
            test is false
        {% endif %}

    {% endblock %}

{% endif %}

<h1>Hello</h1>

    dialog override

        test is false