Symfony version(s) affected

5.4.x, 6.2.x

Description

If you want to use ServiceSubscriberTrait in a class that happens to extend from AbstractController, the following error will appear:

Symfony\Component\ErrorHandler\Error\FatalError:
Compile Error: Declaration of 
Symfony\Contracts\Service\ServiceSubscriberTrait::setContainer(Psr\Container\ContainerInterface $container) 
must be compatible with 
Symfony\Bundle\FrameworkBundle\Controller\AbstractController::setContainer(Psr\Container\ContainerInterface $container): ?Psr\Container\ContainerInterface

  at src/Controller/AppAbstractController.php

The point of ServiceSubscriberTrait::setContainer is that it automatically looks for a setContainer method in the parent class and uses it, if available. However, that's incompatible in this case, as the parent setContainer method has a more specific return type.

How to reproduce

namespace App\Controller;

use Doctrine\DBAL\Connection;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Contracts\Service\ServiceSubscriberTrait;

class AppAbstractController extends AbstractController
{
    use ServiceSubscriberTrait;

    private function databaseConnection(): Connection
    {
        return $this->container->get(__METHOD__);
    }
}

Possible Solution

I did not create a PR right away as I am not sure whether this can technically only be fixed in Symfony 7? But if it's not fixed the ServiceSubscriberTrait is unusable for cases where a parent class actually implements setContainer with a specific return type.

Though may be instead of changing the return type for ServiceSubscriberTrait::setContainer the return type of AbstractController::setContainer should be removed, which should be safe to do in a bugfix release. After all, the ContainerAwareInterface and ContainerAwareTrait also do not use a return type on setContainer.

Additional Context

No response