Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / vendor / symfony / dependency-injection / Tests / Fixtures / xml / services_instanceof_with_parent.xml

<?xml version="1.0" encoding="utf-8"?>
<container xmlns="http://symfony.com/schema/dic/services" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">
    <services>
        <instanceof id="FooInterface" autowire="true" />

        <service id="parent_service" class="stdClass" />
        <service id="child_service" class="stdClass" parent="parent_service" />
    </services>
</container>