src/Symfony/Bundle/FrameworkBundle/Resources/config/form.xml

    <!-- FormTypeValidatorExtension -->
    <service id="form.type_extension.field" class="Symfony\Component\Form\Extension\Validator\Type\FormTypeValidatorExtension">
        <tag name="form.type_extension" alias="field" />
        <argument type="service" id="validator" />
    </service>

    <!-- FormTypeValidatorExtension -->
    <service id="form.type_extension.form" class="Symfony\Component\Form\Extension\Validator\Type\FormTypeValidatorExtension">
        <tag name="form.type_extension" alias="form" />
        <argument type="service" id="validator" />
    </service>