BarService.php

<?php

namespace App;

class BarService
{
    public function bar(): string
    {
        return 'bar';
    }
}

BarServiceDecorator.php

<?php

namespace App;

use ApiPlatform\Core\JsonSchema\TypeFactoryInterface;
use Symfony\Component\DependencyInjection\Attribute\AsDecorator;
use Symfony\Component\DependencyInjection\Attribute\MapDecorated;

#[AsDecorator(decorates: BarService::class)]
class BarServiceDecorator
{
    private $inner;

    public function __construct(#[MapDecorated] $inner)
    {
        $this->inner = $inner;
    }

    public function bar(): string
    {
        return $this->inner->bar() . ' Bar';
    }
}

TestController.php

<?php

namespace App\Controller;

use App\Service\BarService;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Annotation\Route;

class TestController extends AbstractController
{
    /**
     * @Route("/foo")
     */
    public function foo(BarService $barService)
    {
        return new Response('foo ' . $barService->bar());
    }
}

As typehint I use the service which we want to decorate. The example is deliberately without interfaces, as in the Symfony docs.
Without our decorator extending the service class, we get a type hint error.

App\Controller\TestController::foo(): Argument #1 ($barService) must be of type App\Service\BarService, App\Service\BarServiceDecorator given

Isn't the Decorator supposed to wrap the service?
like $this->services[BarService::class] = new BarServiceDecorator(new BarService());
If I debug the Container with php bin/console debug:container App\Service\BarService Then I get the Decorator displayed as a service.

 ---------------- --------------------------------- 
  Option           Value                            
 ---------------- --------------------------------- 
  Service ID       App\Service\BarServiceDecorator  
  Class            App\Service\BarServiceDecorator  
  Tags             -                                
  Public           no                               
  Synthetic        no                               
  Lazy             no                               
  Shared           yes                              
  Abstract         no                               
  Autowired        yes                              
  Autoconfigured   yes                              
 ---------------- --------------------------------- 

Shouldn't it be possible to continue using the original typehint in the controller without Decorator having to extend the class?

If our BarService now implements a BarInterface and I set the interface as type in the controller I get almost the same error.

App\Controller\TestController::foo(): Argument #1 ($barService) must be of type App\Service\BarServiceInterface, App\Service\BarServiceDecorator given.

BarServiceDecorator needs to either extends BarService, or implements BarServiceInterface

BarServiceDecorator needs to either extends BarService, or implements BarServiceInterface

Thanks 👍

Here is another example of how to use the whole thing with interfaces.

<?php

namespace App\Controller;

use App\Service\BarServiceInterface;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Annotation\Route;

class TestController extends AbstractController
{
    /**
     * @Route("/foo")
     */
    public function foo(BarServiceInterface $barService)
    {
        return new Response('foo ' . $barService->bar());
    }
}

<?php

namespace App\Service;

use Symfony\Component\DependencyInjection\Attribute\AsDecorator;
use Symfony\Component\DependencyInjection\Attribute\MapDecorated;

#[AsDecorator(decorates: BarServiceInterface::class)]
class BarServiceDecorator implements BarServiceInterface
{
    private $inner;

    public function __construct(#[MapDecorated] $inner)
    {
        $this->inner = $inner;
    }

    public function bar()
    {
        return $this->inner->bar() . ' BAZ';
    }
}

<?php

namespace App\Service;

interface BarServiceInterface
{
    public function bar();
}

<?php

namespace App\Service;

class BarService implements BarServiceInterface
{
    public function bar()
    {
        return 'bar';
    }
}

    # services.yaml
    App\Service\BarServiceInterface: '@App\Service\BarService'

https://symfony.com/doc/current/service_container/autowiring.html#working-with-interfaces
https://symfony.com/doc/current/service_container/service_decoration.html