<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8" />
    </head>
    <body>
        <div>
            {% if is_granted("IS_AUTHENTICATED_REMEMBERED") %}
                {{ 'layout.logged_in_as'|trans({'%username%': app.user.username}, 'FOSUserBundle') }} |
                <a href="{{ path('fos_user_security_logout') }}">
                    {{ 'layout.logout'|trans({}, 'FOSUserBundle') }}
                </a>
            {% else %}
                <a href="{{ path('fos_user_security_login') }}">{{ 'layout.login'|trans({}, 'FOSUserBundle') }}</a>
            {% endif %}
        </div>

        {% if app.request.hasPreviousSession %}
            {% for type, messages in app.session.flashbag.all() %}
                {% for message in messages %}
                    <div class="flash-{{ type }}">
                        {{ message }}
                    </div>
                {% endfor %}
            {% endfor %}
        {% endif %}

        <div>
            {% block fos_user_content %}
            {% endblock fos_user_content %}
        </div>
    </body>
</html>

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8" />
    </head>
    <body>
        <div>
            {% if is_granted("IS_AUTHENTICATED_REMEMBERED") %}
                {{ 'layout.logged_in_as'|trans({'%username%': app.user.username}, 'FOSUserBundle') }} |
                <a href="{{ path('fos_user_security_logout') }}">
                    {{ 'layout.logout'|trans({}, 'FOSUserBundle') }}
                </a>
            {% else %}
                <a href="{{ path('fos_user_security_login') }}">{{ 'layout.login'|trans({}, 'FOSUserBundle') }}</a>
            {% endif %}
        </div>

        {% if app.request.hasSession and app.request.hasPreviousSession %}
            {% for type, messages in app.session.flashbag.all() %}
                {% for message in messages %}
                    <div class="flash-{{ type }}">
                        {{ message }}
                    </div>
                {% endfor %}
            {% endfor %}
        {% endif %}

        <div>
            {% block fos_user_content %}
            {% endblock fos_user_content %}
        </div>
    </body>
</html>

Check for the presence of the session in the template
Check for the presence of the session in the template
