diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php
index b5c4bba2d516..c40d6f5d3d0f 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveClassPass.php
@@ -13,6 +13,7 @@
 
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\ChildDefinition;
+use Symfony\Component\DependencyInjection\Exception\InvalidArgumentException;
 
 /**
  * @author Nicolas Grekas <p@tchwork.com>
@@ -27,10 +28,13 @@ class ResolveClassPass implements CompilerPassInterface
     public function process(ContainerBuilder $container)
     {
         foreach ($container->getDefinitions() as $id => $definition) {
-            if ($definition instanceof ChildDefinition || $definition->isSynthetic() || null !== $definition->getClass()) {
+            if ($definition->isSynthetic() || null !== $definition->getClass()) {
                 continue;
             }
             if (preg_match('/^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+(?:\\\\[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+)++$/', $id)) {
+                if ($definition instanceof ChildDefinition && !class_exists($id)) {
+                    throw new InvalidArgumentException(sprintf('Service definition "%s" has a parent but no class, and its name looks like a FQCN. Either the class is missing or you want to inherit it from the parent service. To resolve this ambiguity, please rename this service to a non-FQCN (e.g. using dots), or create the missing class.', $id));
+                }
                 $this->changes[strtolower($id)] = $id;
                 $definition->setClass($id);
             }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveClassPassTest.php b/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveClassPassTest.php
index 8a4d99e774d8..acbcf1dda2d8 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveClassPassTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveClassPassTest.php
@@ -12,6 +12,7 @@
 namespace Symfony\Component\DependencyInjection\Tests\Compiler;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\DependencyInjection\ChildDefinition;
 use Symfony\Component\DependencyInjection\Compiler\ResolveClassPass;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Tests\Fixtures\CaseSensitiveClass;
@@ -23,11 +24,10 @@ class ResolveClassPassTest extends TestCase
      */
     public function testResolveClassFromId($serviceId)
     {
-        $pass = new ResolveClassPass();
         $container = new ContainerBuilder();
         $def = $container->register($serviceId);
 
-        $pass->process($container);
+        (new ResolveClassPass())->process($container);
 
         $this->assertSame($serviceId, $def->getClass());
     }
@@ -43,11 +43,10 @@ public function provideValidClassId()
      */
     public function testWontResolveClassFromId($serviceId)
     {
-        $pass = new ResolveClassPass();
         $container = new ContainerBuilder();
         $def = $container->register($serviceId);
 
-        $pass->process($container);
+        (new ResolveClassPass())->process($container);
 
         $this->assertNull($def->getClass());
     }
@@ -58,4 +57,41 @@ public function provideInvalidClassId()
         yield array('bar');
         yield array('\DateTime');
     }
+
+    public function testNonFqcnChildDefinition()
+    {
+        $container = new ContainerBuilder();
+        $parent = $container->register('App\Foo', null);
+        $child = $container->setDefinition('App\Foo.child', new ChildDefinition('App\Foo'));
+
+        (new ResolveClassPass())->process($container);
+
+        $this->assertSame('App\Foo', $parent->getClass());
+        $this->assertNull($child->getClass());
+    }
+
+    public function testClassFoundChildDefinition()
+    {
+        $container = new ContainerBuilder();
+        $parent = $container->register('App\Foo', null);
+        $child = $container->setDefinition(self::class, new ChildDefinition('App\Foo'));
+
+        (new ResolveClassPass())->process($container);
+
+        $this->assertSame('App\Foo', $parent->getClass());
+        $this->assertSame(self::class, $child->getClass());
+    }
+
+    /**
+     * @expectedException \Symfony\Component\DependencyInjection\Exception\InvalidArgumentException
+     * @expectedExceptionMessage Service definition "App\Foo\Child" has a parent but no class, and its name looks like a FQCN. Either the class is missing or you want to inherit it from the parent service. To resolve this ambiguity, please rename this service to a non-FQCN (e.g. using dots), or create the missing class.
+     */
+    public function testAmbiguousChildDefinition()
+    {
+        $container = new ContainerBuilder();
+        $parent = $container->register('App\Foo', null);
+        $child = $container->setDefinition('App\Foo\Child', new ChildDefinition('App\Foo'));
+
+        (new ResolveClassPass())->process($container);
+    }
 }