Merge branch '4.2'

* 4.2:
  the string "0" is a valid service identifier
  remove unreachable code

src/Symfony/Bridge/Twig/Tests/Node/TransNodeTest.php

@@ -49,12 +49,8 @@ class TransNodeTest extends TestCase
 
     protected function getVariableGetterWithStrictCheck($name)
     {
-        if (Environment::VERSION_ID > 20404) {
-            return sprintf('(isset($context["%s"]) || array_key_exists("%1$s", $context) ? $context["%1$s"] : (function () { throw new Twig_Error_Runtime(\'Variable "%1$s" does not exist.\', 0, $this->source); })())', $name);
-        }
-
         if (Environment::MAJOR_VERSION >= 2) {
-            return sprintf('(isset($context["%s"]) || array_key_exists("%1$s", $context) ? $context["%1$s"] : (function () { throw new Twig_Error_Runtime(\'Variable "%1$s" does not exist.\', 0, $this->getSourceContext()); })())', $name);
+            return sprintf('(isset($context["%s"]) || array_key_exists("%1$s", $context) ? $context["%1$s"] : (function () { throw new Twig_Error_Runtime(\'Variable "%1$s" does not exist.\', 0, $this->source); })())', $name);
         }
 
         return sprintf('($context["%s"] ?? $this->getContext($context, "%1$s"))', $name);

src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php

@@ -497,7 +497,7 @@ class XmlFileLoader extends FileLoader
 
             switch ($arg->getAttribute('type')) {
                 case 'service':
-                    if (!$arg->getAttribute('id')) {
+                    if ('' === $arg->getAttribute('id')) {
                         throw new InvalidArgumentException(sprintf('Tag "<%s>" with type="service" has no or empty "id" attribute in "%s".', $name, $file));
                     }
 
@@ -561,7 +561,7 @@ class XmlFileLoader extends FileLoader
      * @param \DOMNode $node
      * @param mixed    $name
      *
-     * @return array
+     * @return \DOMElement[]
      */
     private function getChildren(\DOMNode $node, $name)
     {

src/Symfony/Component/DependencyInjection/Tests/Fixtures/xml/services6.xml

@@ -61,5 +61,9 @@
     </service>
     <service id="alias_for_foo" alias="foo" />
     <service id="another_alias_for_foo" alias="foo" public="false" />
+    <service id="0" class="FooClass" />
+    <service id="1" class="FooClass">
+      <argument type="service" id="0" />
+    </service>
   </services>
 </container>