synthetic_service.php

<?php

use Symfony\Component\DependencyInjection\ContainerBuilder;

$container = new ContainerBuilder();

// ... set up the container

// might be helpful to check if the service is synthetic
var_dump($container->getDefinition('my.synthetic.service')->isSynthetic()); // true

if ($container->hasDefinition('my.synthetic.service')) {
	$container->set('my.synthetic.service', new SyntheticService());
}

var_dump($container->hasDefinition('my.synthetic.service')); // false

This is a bad work with "Synthetic" mode of service.