[DependencyInjection] throw proper exception when decorating a synthetic service

src/Symfony/Component/DependencyInjection/Compiler/DecoratorServicePass.php

@@ -14,6 +14,7 @@ namespace Symfony\Component\DependencyInjection\Compiler;
 use Symfony\Component\DependencyInjection\Alias;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\DependencyInjection\Exception\InvalidArgumentException;
 use Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException;
 use Symfony\Component\DependencyInjection\Reference;
 
@@ -59,6 +60,7 @@ class DecoratorServicePass implements CompilerPassInterface
                 $public = $alias->isPublic();
                 $private = $alias->isPrivate();
                 $container->setAlias($renamedId, new Alias((string) $alias, false));
+                $decoratedDefinition = $container->findDefinition($alias);
             } elseif ($container->hasDefinition($inner)) {
                 $decoratedDefinition = $container->getDefinition($inner);
                 $public = $decoratedDefinition->isPublic();
@@ -72,10 +74,15 @@ class DecoratorServicePass implements CompilerPassInterface
             } elseif (ContainerInterface::NULL_ON_INVALID_REFERENCE === $invalidBehavior) {
                 $public = $definition->isPublic();
                 $private = $definition->isPrivate();
+                $decoratedDefinition = null;
             } else {
                 throw new ServiceNotFoundException($inner, $id);
             }
 
+            if ($decoratedDefinition && $decoratedDefinition->isSynthetic()) {
+                throw new InvalidArgumentException(sprintf('A synthetic service cannot be decorated: service "%s" cannot decorate "%s".', $id, $inner));
+            }
+
             if (isset($decoratingDefinitions[$inner])) {
                 $decoratingDefinition = $decoratingDefinitions[$inner];
 

src/Symfony/Component/DependencyInjection/Tests/Compiler/DecoratorServicePassTest.php

@@ -16,6 +16,7 @@ use Symfony\Component\DependencyInjection\Alias;
 use Symfony\Component\DependencyInjection\Compiler\DecoratorServicePass;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\DependencyInjection\Exception\InvalidArgumentException;
 use Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException;
 
 class DecoratorServicePassTest extends TestCase
@@ -262,6 +263,23 @@ class DecoratorServicePassTest extends TestCase
         $this->assertEquals(['bar' => ['attr' => 'baz'], 'foobar' => ['attr' => 'bar']], $container->getDefinition('baz')->getTags());
     }
 
+    public function testCannotDecorateSyntheticService()
+    {
+        $container = new ContainerBuilder();
+        $container
+            ->register('foo')
+            ->setSynthetic(true)
+        ;
+        $container
+            ->register('baz')
+            ->setDecoratedService('foo')
+        ;
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('A synthetic service cannot be decorated: service "baz" cannot decorate "foo".');
+        $this->process($container);
+    }
+
     protected function process(ContainerBuilder $container)
     {
         $repeatedPass = new DecoratorServicePass();