diff --git a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
index 46fcdc873ea0e..9ebfb27322d68 100644
--- a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
+++ b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
@@ -759,6 +759,10 @@ public function setDefinition($id, Definition $definition)
             throw new BadMethodCallException('Adding definition to a frozen container is not allowed');
         }
 
+        if (null === $definition->getClass() && class_exists($id)) {
+            $definition->setClass($id);
+        }
+
         $id = strtolower($id);
 
         unset($this->aliasDefinitions[$id]);
diff --git a/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php b/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
index e59b0826b5631..f80d5e5f3063d 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php
@@ -29,6 +29,7 @@
 use Symfony\Component\DependencyInjection\ParameterBag\EnvPlaceholderParameterBag;
 use Symfony\Component\Config\Resource\FileResource;
 use Symfony\Component\DependencyInjection\Tests\Fixtures\CustomDefinition;
+use Symfony\Component\DependencyInjection\Tests\Fixtures\CaseSensitiveClass;
 use Symfony\Component\ExpressionLanguage\Expression;
 
 class ContainerBuilderTest extends \PHPUnit_Framework_TestCase
@@ -808,6 +809,19 @@ public function testAutowiring()
 
         $this->assertEquals('a', (string) $container->getDefinition('b')->getArgument(0));
     }
+
+    public function testClassFromId()
+    {
+        $container = new ContainerBuilder();
+
+        $unknown = $container->register('unknown_class');
+        $class = $container->register(\stdClass::class);
+        $autoloadClass = $container->register(CaseSensitiveClass::class);
+
+        $this->assertNull($unknown->getClass());
+        $this->assertEquals(\stdClass::class, $class->getClass());
+        $this->assertEquals(CaseSensitiveClass::class, $autoloadClass->getClass());
+    }
 }
 
 class FooClass
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/CaseSensitiveClass.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/CaseSensitiveClass.php
new file mode 100644
index 0000000000000..87b258429e6a3
--- /dev/null
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/CaseSensitiveClass.php
@@ -0,0 +1,16 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\DependencyInjection\Tests\Fixtures;
+
+class CaseSensitiveClass
+{
+}
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/xml/class_from_id.xml b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/xml/class_from_id.xml
new file mode 100644
index 0000000000000..45415cce472f0
--- /dev/null
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/xml/class_from_id.xml
@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<container xmlns="http://symfony.com/schema/dic/services" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">
+  <services>
+    <service id="Symfony\Component\DependencyInjection\Tests\Fixtures\CaseSensitiveClass" />
+  </services>
+</container>
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/class_from_id.yml b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/class_from_id.yml
new file mode 100644
index 0000000000000..33f0b2ea6821f
--- /dev/null
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/class_from_id.yml
@@ -0,0 +1,3 @@
+services:
+    Symfony\Component\DependencyInjection\Tests\Fixtures\CaseSensitiveClass:
+      autowire: true
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Loader/XmlFileLoaderTest.php b/src/Symfony/Component/DependencyInjection/Tests/Loader/XmlFileLoaderTest.php
index 5a166d963dc67..96159a3acea48 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Loader/XmlFileLoaderTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Loader/XmlFileLoaderTest.php
@@ -19,6 +19,7 @@
 use Symfony\Component\DependencyInjection\Loader\IniFileLoader;
 use Symfony\Component\Config\Loader\LoaderResolver;
 use Symfony\Component\Config\FileLocator;
+use Symfony\Component\DependencyInjection\Tests\Fixtures\CaseSensitiveClass;
 use Symfony\Component\ExpressionLanguage\Expression;
 
 class XmlFileLoaderTest extends \PHPUnit_Framework_TestCase
@@ -556,6 +557,15 @@ public function testAutowire()
         $this->assertTrue($container->getDefinition('bar')->isAutowired());
     }
 
+    public function testClassFromId()
+    {
+        $container = new ContainerBuilder();
+        $loader = new XmlFileLoader($container, new FileLocator(self::$fixturesPath.'/xml'));
+        $loader->load('class_from_id.xml');
+
+        $this->assertEquals(CaseSensitiveClass::class, $container->getDefinition(CaseSensitiveClass::class)->getClass());
+    }
+
     /**
      * @group legacy
      * @expectedDeprecation Using the attribute "class" is deprecated for the service "bar" which is defined as an alias %s.
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php b/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
index fabaa4859c578..19861c2524d4c 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
@@ -19,6 +19,7 @@
 use Symfony\Component\DependencyInjection\Loader\PhpFileLoader;
 use Symfony\Component\Config\Loader\LoaderResolver;
 use Symfony\Component\Config\FileLocator;
+use Symfony\Component\DependencyInjection\Tests\Fixtures\CaseSensitiveClass;
 use Symfony\Component\ExpressionLanguage\Expression;
 
 class YamlFileLoaderTest extends \PHPUnit_Framework_TestCase
@@ -326,6 +327,15 @@ public function testAutowire()
         $this->assertTrue($container->getDefinition('bar_service')->isAutowired());
     }
 
+    public function testClassFromId()
+    {
+        $container = new ContainerBuilder();
+        $loader = new YamlFileLoader($container, new FileLocator(self::$fixturesPath.'/yaml'));
+        $loader->load('class_from_id.yml');
+
+        $this->assertEquals(CaseSensitiveClass::class, $container->getDefinition(CaseSensitiveClass::class)->getClass());
+    }
+
     /**
      * @expectedException \Symfony\Component\DependencyInjection\Exception\InvalidArgumentException
      * @expectedExceptionMessage The value of the "decorates" option for the "bar" service must be the id of the service without the "@" prefix (replace "@foo" with "foo").