…ause the name is already in use (ogizanagi)This PR was merged into the 3.3-dev branch.Discussion----------[DI] Fix Cannot declare class ...\DefinitionDecorator, because the name is already in use| Q             | A| ------------- | ---| Branch?       | 3.3| Bug fix?      | yes| New feature?  | no| BC breaks?    | no| Deprecations? | no| Tests pass?   | yes| Fixed tickets |#21369| License       | MIT| Doc PR        | N/AThe `return` trick doesn't seem to work, and php is still trying to declare the `DefinitionDecorator` class, which causes the "Cannot declare class ...\DefinitionDecorator, because the name is already in use" error because of the `class_alias` previously declared in `ChildDefinition.php`.This never happens as soon as the `ChildDefinition` class is used first, as the alias will take hand, but their are some situations, like in some unit test cases it can happen apparently, because `DefinitionDecorator` is used first.Commits-------530849e [DI] Fix Cannot declare class ...\DefinitionDecorator, because the name is already in use

…sDecisionManager BC layer (ogizanagi)This PR was merged into the 3.3-dev branch.Discussion----------[Security] Fix TraceableAccessDecisionManager / DebugAccessDecisionManager BC layer| Q             | A| ------------- | ---| Branch?       | 3.3| Bug fix?      | yes| New feature?  | no| BC breaks?    | no| Deprecations? | no| Tests pass?   | yes| Fixed tickets |#22657 (comment)| License       | MIT| Doc PR        | N/ASame as#22657 for the renaming of `DebugAccessDecisionManager` into `TraceableAccessDecisionManager`. Indeed, I think we also require to redeclare the old `DebugAccessDecisionManager` for composer `classmap-authoritative` autoloading strategy & classmap dumper.AppVeyor failures unrelated.Edit: Re-thinking about it, it's probably not very common to dump the classmap and use the `classmap-authoritative` strategy when using those classes... That's to say: in debug mode/dev env. So it may be the reason why the class wasn't redeclared on contrary of `DefinitionDecorator`.Commits-------5b123b9 [Security] Fix TraceableAccessDecisionManager / DebugAccessDecisionManager BC layer

This PR was merged into the 1.x branch.Discussion----------Use class_exists instead of requireI think this is required forsymfony/symfony#23073 to pass. It's probably very similar tosymfony/symfony#22657.A simple reproducer:```php<?phpuse Twig\Node\Node;require_once __DIR__ . '/vendor/autoload.php';new Node();new \Twig_Node();```[Without this patch](https://travis-ci.org/symfony/symfony/jobs/239712676#L2100):```phpPHP Fatal error:  Cannot declare class Twig_Node, because the name is already in use in vendor/twig/twig/lib/Twig/Node.php on line 20```With, everything works fine, whatever the two calls order.Commits-------2c174e4 Use class_exists instead of require