I am also able to reproduce it by using https://github.com/dunglas/symfony-docker as a skeleton setup, then changing the default routes.yaml to:

controllers:
    resource: ../src/Controller/
    type: attribute
    prefix:
        nl_NL: /nl

And adding a simple controller in src/Controller/Controller.php:

<?php declare(strict_types=1);

namespace App\Controller;

use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Attribute\Route;
use Symfony\Component\Routing\RouterInterface;

class Controller
{
    #[Route(path: [
        'nl_NL' => '/',
    ], name: 'default', alias: ['my-alias'])]
    public function __invoke(RouterInterface $router): Response
    {
        return new Response($router->generate('my-alias'));

        // this also fails with the same exception
        return new Response($router->generate('default'));
    }
}

Which throws

Target route "default" for alias "my-alias" does not exist.

If I remove the dev routes loaded by the framework bundle, then dump the CompiledUrlGeneratorDumper.php variables before it actually fails, I have the following, which shows why it fails:

// dump($this->getRoutes()->all());

^ array:1 [
  "default.nl_NL" => Symfony\Component\Routing\Route^ {#710
    -path: "/nl/"
    -host: ""
    -schemes: []
    -methods: []
    -defaults: array:3 [
      "_controller" => "App\Controller\Controller"
      "_locale" => "nl_NL"
      "_canonical_route" => "default"
    ]
    -requirements: []
    -options: array:2 [
      "compiler_class" => "Symfony\Component\Routing\RouteCompiler"
      "utf8" => true
    ]
    -condition: ""
    -compiled: Symfony\Component\Routing\CompiledRoute^ {#2761
      -staticPrefix: "/nl/"
      -regex: "{^/nl/$}sDu"
      -tokens: array:1 [
        0 => array:2 [
          0 => "text"
          1 => "/nl/"
        ]
      ]
      -pathVariables: []
      -hostRegex: null
      -hostTokens: []
      -hostVariables: []
      -variables: []
    }
  }
]

// dump($this->getRoutes()->getAliases());

^ array:3 [
  "my-alias" => Symfony\Component\Routing\Alias^ {#5056
    -deprecation: []
    -id: "default"
  }
  "App\Controller\Controller::__invoke" => Symfony\Component\Routing\Alias^ {#4763
    -deprecation: []
    -id: "default.nl_NL"
  }
  "App\Controller\Controller" => Symfony\Component\Routing\Alias^ {#4978
    -deprecation: []
    -id: "default.nl_NL"
  }
]

So it looks like the auto generated aliases point at the right (dynamically generated) route name, but the alias from the #[Route] attribute points at the hardcoded route name.

Looks like the implementation of the alias property of the attribute has forgotten to consider the case of localized routes.

Hi @alcohol,

Indeed, localized routes wasn't something I was aware of when I started my PR.
Aliases in route attribute relies on the name of #[Route] Attribute to create the link between the alias and the route itself. But, with localized routes, it does not work because the route name in the router is default.en_US/default.fr_FR/default.nl_NL...etc.

I understand that. But the AttributeClassLoader is aware of those as far as I can tell, so in theory this should be relatively easy to add.

Wouldn't something like the following suffice?

diff --git i/src/Symfony/Component/Routing/Loader/AttributeClassLoader.php w/src/Symfony/Component/Routing/Loader/AttributeClassLoader.php
index 254582bf35..6b0f4e8e88 100644
--- i/src/Symfony/Component/Routing/Loader/AttributeClassLoader.php
+++ w/src/Symfony/Component/Routing/Loader/AttributeClassLoader.php
@@ -242,8 +242,13 @@ abstract class AttributeClassLoader implements LoaderInterface
                 $collection->add($name, $route, $priority);
             }
             foreach ($attr->getAliases() as $aliasAttribute) {
+                $destination = $name;
+                if (0 !== $locale) {
+                    $destination .= '.'.$locale;
+                }
+
                 if ($aliasAttribute instanceof DeprecatedAlias) {
-                    $alias = $collection->addAlias($aliasAttribute->getAliasName(), $name);
+                    $alias = $collection->addAlias($aliasAttribute->getAliasName(), $destination);
                     $alias->setDeprecated(
                         $aliasAttribute->getPackage(),
                         $aliasAttribute->getVersion(),
@@ -252,7 +257,7 @@ abstract class AttributeClassLoader implements LoaderInterface
                     continue;
                 }

-                $collection->addAlias($aliasAttribute, $name);
+                $collection->addAlias($aliasAttribute, $destination);
             }
         }
     }

Ah rats, nevermind. Trying to write out a test scenario I actually discovered that this is probably not possible because the alias will only point to the last added locale of a route. There cannot be an alias for each localized entry because the alias entry is not localized and has to be unique.

I've come to the same conclusion, same error when using attribute or YAML. So this is more a feature than a bug to me.

Handling locales in aliases might be a cool feature, but I don't know if it is relevant for the community.

Should the code throw an exception instead then? Since using an alias on a localized route either does not work, or does not work as expected (only aliasing the last defined locale).

I've come to the same conclusion, same error when using attribute or YAML. So this is more a feature than a bug to me.

Are you sure about this by the way? I thought it does work for yaml, because it uses a different approach to load localized routes.

Ah just tried it, does not work indeed:

default:
    controller: App\Controller\Controller
    prefix:
        nl_NL: /nl
    path:
        nl_NL: /
my_alias:
    alias: default

In CompiledUrlGeneratorDumper.php line 72:
                                                               
  Target route "default" for alias "my_alias" does not exist.  

@stof @xabbuh do you see any viable path forward here?

I am not that deep into the code of the Routing component but since for one localized route there are several routes behind the scenes (for default there are default.en_US and default.fr_FR) I wonder if this couldn't be reflected for the alias to something like my_alias.en_US pointing to default.en_US and my_alias.fr_FR pointing to default.fr_FR.

Well, in theory, yes. But you would not reference an alias that way in your code or twig templates, right? You would just call it my_alias. Then again, you also just use default to generate a route in code or twig templates. So perhaps the same resolving behind that could be used for aliases? Alternatively, we could take into account the canonical that we set on the localized routes maybe?

Well, in theory, yes. But you would not reference an alias that way in your code or twig templates, right? You would just call it my_alias. Then again, you also just use default to generate a route in code or twig templates.

That's exactly what I had in mind. But again I am not so deep into the Routing component's code to judge whether that's possible.

If aliases don't work for other formats either, I suggest we document that limitation (and maybe add a better exception if possible) for 7.3, and that finding a way to support localized aliases gets handled as a new feature in a next minor version.