File: vendor/symfony/symfony/src/Symfony/Component/Form/Resources/config/validation.xml

<?xml version="1.0" ?>

<constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping http://symfony.com/schema/dic/constraint-mapping/constraint-mapping-1.0.xsd">

  <class name="Symfony\Component\Form\Form">
    <constraint name="Symfony\Component\Form\Extension\Validator\Constraints\Form" />
    <property name="children">
        <constraint name="Valid" />
    </property>
  </class>
</constraint-mapping>