diff --git a/src/Symfony/Component/DependencyInjection/ChildDefinition.php b/src/Symfony/Component/DependencyInjection/ChildDefinition.php
index 53a1d188a0..b157642bd3 100644
--- a/src/Symfony/Component/DependencyInjection/ChildDefinition.php
+++ b/src/Symfony/Component/DependencyInjection/ChildDefinition.php
@@ -14,8 +14,6 @@ namespace Symfony\Component\DependencyInjection;
 use Symfony\Component\DependencyInjection\Exception\InvalidArgumentException;
 use Symfony\Component\DependencyInjection\Exception\OutOfBoundsException;
 
-class_alias(ChildDefinition::class, DefinitionDecorator::class);
-
 /**
  * This definition extends another definition.
  *
@@ -199,3 +197,5 @@ class ChildDefinition extends Definition
         return $this;
     }
 }
+
+class_alias(ChildDefinition::class, DefinitionDecorator::class);
diff --git a/src/Symfony/Component/DependencyInjection/Tests/ChildDefinitionTest.php b/src/Symfony/Component/DependencyInjection/Tests/ChildDefinitionTest.php
index 8f89e3b626..1b84cf90bb 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/ChildDefinitionTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/ChildDefinitionTest.php
@@ -12,6 +12,7 @@
 namespace Symfony\Component\DependencyInjection\Tests;
 
 use Symfony\Component\DependencyInjection\ChildDefinition;
+use Symfony\Component\DependencyInjection\DefinitionDecorator;
 
 class ChildDefinitionTest extends \PHPUnit_Framework_TestCase
 {
@@ -125,4 +126,9 @@ class ChildDefinitionTest extends \PHPUnit_Framework_TestCase
 
         $def->getArgument(1);
     }
+
+    public function testDefinitionDecoratorAliasExistsForBackwardsCompatibility()
+    {
+        $this->assertInstanceOf(ChildDefinition::class, new DefinitionDecorator('foo'));
+    }
 }