diff --git a/lib/Twig/BaseNodeVisitor.php b/lib/Twig/BaseNodeVisitor.php
index 30cd2beb5..fac1ec819 100644
--- a/lib/Twig/BaseNodeVisitor.php
+++ b/lib/Twig/BaseNodeVisitor.php
@@ -9,6 +9,9 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Environment.php';
+require_once __DIR__.'/Node.php';
+
 /**
  * Twig_BaseNodeVisitor can be used to make node visitors compatible with Twig 1.x and 2.x.
  *
diff --git a/lib/Twig/Compiler.php b/lib/Twig/Compiler.php
index 2da8c4bfd..37584f2b0 100644
--- a/lib/Twig/Compiler.php
+++ b/lib/Twig/Compiler.php
@@ -10,6 +10,8 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Node.php';
+
 /**
  * Compiles a node to PHP code.
  *
diff --git a/lib/Twig/Environment.php b/lib/Twig/Environment.php
index a41e5cb8d..c04127a7d 100644
--- a/lib/Twig/Environment.php
+++ b/lib/Twig/Environment.php
@@ -758,7 +758,7 @@ class Twig_Environment
 
     public function setLoader(Twig_LoaderInterface $loader)
     {
-        if (!$loader instanceof Twig_SourceContextLoaderInterface && 0 !== strpos(get_class($loader), 'Mock_Twig_LoaderInterface')) {
+        if (!$loader instanceof Twig_SourceContextLoaderInterface && 0 !== strpos(get_class($loader), 'Mock_')) {
             @trigger_error(sprintf('Twig loader "%s" should implement Twig_SourceContextLoaderInterface since version 1.27.', get_class($loader)), E_USER_DEPRECATED);
         }
 
@@ -831,6 +831,12 @@ class Twig_Environment
     public function hasExtension($class)
     {
         $class = ltrim($class, '\\');
+        if (!isset($this->extensionsByClass[$class]) && class_exists($class, false)) {
+            // For BC/FC with namespaced aliases
+            $class = new ReflectionClass($class);
+            $class = $class->name;
+        }
+
         if (isset($this->extensions[$class])) {
             if ($class !== get_class($this->extensions[$class])) {
                 @trigger_error(sprintf('Referencing the "%s" extension by its name (defined by getName()) is deprecated since 1.26 and will be removed in Twig 2.0. Use the Fully Qualified Extension Class Name instead.', $class), E_USER_DEPRECATED);
@@ -860,6 +866,11 @@ class Twig_Environment
     public function getExtension($class)
     {
         $class = ltrim($class, '\\');
+        if (!isset($this->extensionsByClass[$class]) && class_exists($class, false)) {
+            // For BC/FC with namespaced aliases
+            $class = new ReflectionClass($class);
+            $class = $class->name;
+        }
 
         if (isset($this->extensions[$class])) {
             if ($class !== get_class($this->extensions[$class])) {
@@ -938,6 +949,12 @@ class Twig_Environment
         }
 
         $class = ltrim($name, '\\');
+        if (!isset($this->extensionsByClass[$class]) && class_exists($class, false)) {
+            // For BC/FC with namespaced aliases
+            $class = new ReflectionClass($class);
+            $class = $class->name;
+        }
+
         if (isset($this->extensions[$class])) {
             if ($class !== get_class($this->extensions[$class])) {
                 @trigger_error(sprintf('Referencing the "%s" extension by its name (defined by getName()) is deprecated since 1.26 and will be removed in Twig 2.0. Use the Fully Qualified Extension Class Name instead.', $class), E_USER_DEPRECATED);
@@ -1560,4 +1577,4 @@ class Twig_Environment
     }
 }
 
-class_alias('Twig_Environment', 'Twig\Environment', false);
+class_alias('Twig_Environment', 'Twig\Twig', false);
diff --git a/lib/Twig/Error.php b/lib/Twig/Error.php
index 551304588..763a47474 100644
--- a/lib/Twig/Error.php
+++ b/lib/Twig/Error.php
@@ -9,6 +9,8 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Source.php';
+
 /**
  * Twig base exception.
  *
diff --git a/lib/Twig/ExistsLoaderInterface.php b/lib/Twig/ExistsLoaderInterface.php
index 553fb4e5e..968cb21a4 100644
--- a/lib/Twig/ExistsLoaderInterface.php
+++ b/lib/Twig/ExistsLoaderInterface.php
@@ -27,3 +27,5 @@ interface Twig_ExistsLoaderInterface
      */
     public function exists($name);
 }
+
+class_alias('Twig_ExistsLoaderInterface', 'Twig\Loader\ExistsLoaderInterface', false);
diff --git a/lib/Twig/Extension.php b/lib/Twig/Extension.php
index f1c01b7d1..20a794508 100644
--- a/lib/Twig/Extension.php
+++ b/lib/Twig/Extension.php
@@ -8,6 +8,9 @@
  * For the full copyright and license information, please view the LICENSE
  * file that was distributed with this source code.
  */
+
+require_once __DIR__.'/Environment.php';
+
 abstract class Twig_Extension implements Twig_ExtensionInterface
 {
     /**
diff --git a/lib/Twig/Extension/GlobalsInterface.php b/lib/Twig/Extension/GlobalsInterface.php
index 5370b8e2a..922cd2c93 100644
--- a/lib/Twig/Extension/GlobalsInterface.php
+++ b/lib/Twig/Extension/GlobalsInterface.php
@@ -20,3 +20,5 @@
 interface Twig_Extension_GlobalsInterface
 {
 }
+
+class_alias('Twig_Extension_GlobalsInterface', 'Twig\Extension\GlobalsInterface', false);
diff --git a/lib/Twig/Extension/InitRuntimeInterface.php b/lib/Twig/Extension/InitRuntimeInterface.php
index 7a075822f..1549862f4 100644
--- a/lib/Twig/Extension/InitRuntimeInterface.php
+++ b/lib/Twig/Extension/InitRuntimeInterface.php
@@ -20,3 +20,5 @@
 interface Twig_Extension_InitRuntimeInterface
 {
 }
+
+class_alias('Twig_Extension_InitRuntimeInterface', 'Twig\Extension\InitRuntimeInterface', false);
diff --git a/lib/Twig/Extension/Profiler.php b/lib/Twig/Extension/Profiler.php
index b81473c59..14aaf4eee 100644
--- a/lib/Twig/Extension/Profiler.php
+++ b/lib/Twig/Extension/Profiler.php
@@ -9,6 +9,8 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/../Profiler/Profile.php';
+
 class Twig_Extension_Profiler extends Twig_Extension
 {
     private $actives = array();
diff --git a/lib/Twig/ExtensionInterface.php b/lib/Twig/ExtensionInterface.php
index 81e11b494..c029a13d2 100644
--- a/lib/Twig/ExtensionInterface.php
+++ b/lib/Twig/ExtensionInterface.php
@@ -9,6 +9,8 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Environment.php';
+
 /**
  * Interface implemented by extension classes.
  *
diff --git a/lib/Twig/Node.php b/lib/Twig/Node.php
index d1f59e0b0..9251b0839 100644
--- a/lib/Twig/Node.php
+++ b/lib/Twig/Node.php
@@ -10,6 +10,8 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Compiler.php';
+
 /**
  * Represents a node in the AST.
  *
diff --git a/lib/Twig/NodeVisitorInterface.php b/lib/Twig/NodeVisitorInterface.php
index 0172dd90b..9879b5e5a 100644
--- a/lib/Twig/NodeVisitorInterface.php
+++ b/lib/Twig/NodeVisitorInterface.php
@@ -9,6 +9,9 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Environment.php';
+require_once __DIR__.'/Node.php';
+
 /**
  * Twig_NodeVisitorInterface is the interface the all node visitor classes must implement.
  *
diff --git a/lib/Twig/Parser.php b/lib/Twig/Parser.php
index 27018e28b..d8bb4d2f0 100644
--- a/lib/Twig/Parser.php
+++ b/lib/Twig/Parser.php
@@ -10,6 +10,9 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Node.php';
+require_once __DIR__.'/TokenStream.php';
+
 /**
  * Default parser implementation.
  *
diff --git a/lib/Twig/SourceContextLoaderInterface.php b/lib/Twig/SourceContextLoaderInterface.php
index acf21e378..a6e8c4255 100644
--- a/lib/Twig/SourceContextLoaderInterface.php
+++ b/lib/Twig/SourceContextLoaderInterface.php
@@ -29,3 +29,5 @@ interface Twig_SourceContextLoaderInterface
      */
     public function getSourceContext($name);
 }
+
+class_alias('Twig_SourceContextLoaderInterface', 'Twig\Loader\SourceContextLoaderInterface', false);
diff --git a/lib/Twig/Test/NodeTestCase.php b/lib/Twig/Test/NodeTestCase.php
index 2ef14de5c..35d0686bd 100644
--- a/lib/Twig/Test/NodeTestCase.php
+++ b/lib/Twig/Test/NodeTestCase.php
@@ -1,7 +1,5 @@
 <?php
 
-use PHPUnit\Framework\TestCase;
-
 /*
  * This file is part of Twig.
  *
@@ -10,6 +8,12 @@ use PHPUnit\Framework\TestCase;
  * For the full copyright and license information, please view the LICENSE
  * file that was distributed with this source code.
  */
+
+require_once __DIR__.'/../Environment.php';
+require_once __DIR__.'/../Node.php';
+
+use PHPUnit\Framework\TestCase;
+
 abstract class Twig_Test_NodeTestCase extends TestCase
 {
     abstract public function getTests();
diff --git a/lib/Twig/TokenParserInterface.php b/lib/Twig/TokenParserInterface.php
index 0ea3a4e0c..cd559f987 100644
--- a/lib/Twig/TokenParserInterface.php
+++ b/lib/Twig/TokenParserInterface.php
@@ -9,6 +9,9 @@
  * file that was distributed with this source code.
  */
 
+require_once __DIR__.'/Parser.php';
+require_once __DIR__.'/Token.php';
+
 /**
  * Interface implemented by token parsers.
  *
diff --git a/src/Cache/CacheInterface.php b/src/Cache/CacheInterface.php
index ef0401215..68eefff14 100644
--- a/src/Cache/CacheInterface.php
+++ b/src/Cache/CacheInterface.php
@@ -5,7 +5,7 @@ namespace Twig\Cache;
 require __DIR__.'/../../lib/Twig/CacheInterface.php';
 
 if (\false) {
-    class CacheInterface extends \Twig_CacheInterface
+    interface CacheInterface extends \Twig_CacheInterface
     {
     }
 }
diff --git a/src/Extension/ExtensionInterface.php b/src/Extension/ExtensionInterface.php
index 311233da6..13655f832 100644
--- a/src/Extension/ExtensionInterface.php
+++ b/src/Extension/ExtensionInterface.php
@@ -5,7 +5,7 @@ namespace Twig\Extension;
 require __DIR__.'/../../lib/Twig/ExtensionInterface.php';
 
 if (\false) {
-    class ExtensionInterface extends \Twig_ExtensionInterface
+    interface ExtensionInterface extends \Twig_ExtensionInterface
     {
     }
 }
diff --git a/src/Extension/GlobalsInterface.php b/src/Extension/GlobalsInterface.php
new file mode 100644
index 000000000..2310e0826
--- /dev/null
+++ b/src/Extension/GlobalsInterface.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Twig\Extension;
+
+require __DIR__.'/../../lib/Twig/Extension/GlobalsInterface.php';
+
+if (\false) {
+    interface GlobalsInterface extends \Twig_Extension_ExtensionInterface
+    {
+    }
+}
diff --git a/src/Extension/InitRuntimeInterface.php b/src/Extension/InitRuntimeInterface.php
new file mode 100644
index 000000000..4b491e795
--- /dev/null
+++ b/src/Extension/InitRuntimeInterface.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Twig\Extension;
+
+require __DIR__.'/../../lib/Twig/Extension/InitRuntimeInterface.php';
+
+if (\false) {
+    interface InitRuntimeInterface extends \Twig_Extension_InitRuntimeInterface
+    {
+    }
+}
diff --git a/src/Loader/ExistsLoaderInterface.php b/src/Loader/ExistsLoaderInterface.php
new file mode 100644
index 000000000..05dd164b4
--- /dev/null
+++ b/src/Loader/ExistsLoaderInterface.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Twig\Loader;
+
+require __DIR__.'/../../lib/Twig/ExistsLoaderInterface.php';
+
+if (\false) {
+    interface ExistsLoaderInterface extends \Twig_ExistsLoaderInterface
+    {
+    }
+}
diff --git a/src/Loader/LoaderInterface.php b/src/Loader/LoaderInterface.php
index b4843ae89..14141668d 100644
--- a/src/Loader/LoaderInterface.php
+++ b/src/Loader/LoaderInterface.php
@@ -5,7 +5,7 @@ namespace Twig\Loader;
 require __DIR__.'/../../lib/Twig/LoaderInterface.php';
 
 if (\false) {
-    class LoaderInterface extends \Twig_LoaderInterface
+    interface LoaderInterface extends \Twig_LoaderInterface
     {
     }
 }
diff --git a/src/Loader/SourceContextLoaderInterface.php b/src/Loader/SourceContextLoaderInterface.php
new file mode 100644
index 000000000..79bb35eec
--- /dev/null
+++ b/src/Loader/SourceContextLoaderInterface.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Twig\Loader;
+
+require __DIR__.'/../../lib/Twig/SourceContextLoaderInterface.php';
+
+if (\false) {
+    interface SourceContextLoaderInterface extends \Twig_SourceContextLoaderInterface
+    {
+    }
+}
diff --git a/src/Node/NodeCaptureInterface.php b/src/Node/NodeCaptureInterface.php
index 90700408c..c93eb42bf 100644
--- a/src/Node/NodeCaptureInterface.php
+++ b/src/Node/NodeCaptureInterface.php
@@ -5,7 +5,7 @@ namespace Twig\Node;
 require __DIR__.'/../../lib/Twig/NodeCaptureInterface.php';
 
 if (\false) {
-    class NodeCaptureInterface extends \Twig_NodeCaptureInterface
+    interface NodeCaptureInterface extends \Twig_NodeCaptureInterface
     {
     }
 }
diff --git a/src/Node/NodeOutputInterface.php b/src/Node/NodeOutputInterface.php
index b3a4e0b4c..f1085d963 100644
--- a/src/Node/NodeOutputInterface.php
+++ b/src/Node/NodeOutputInterface.php
@@ -5,7 +5,7 @@ namespace Twig\Node;
 require __DIR__.'/../../lib/Twig/NodeOutputInterface.php';
 
 if (\false) {
-    class NodeOutputInterface extends \Twig_NodeOutputInterface
+    interface NodeOutputInterface extends \Twig_NodeOutputInterface
     {
     }
 }
diff --git a/src/NodeVisitor/NodeVisitorInterface.php b/src/NodeVisitor/NodeVisitorInterface.php
index d45d3b2bd..65d87b768 100644
--- a/src/NodeVisitor/NodeVisitorInterface.php
+++ b/src/NodeVisitor/NodeVisitorInterface.php
@@ -5,7 +5,7 @@ namespace Twig\NodeVisitor;
 require __DIR__.'/../../lib/Twig/NodeVisitorInterface.php';
 
 if (\false) {
-    class NodeVisitorInterface extends \Twig_NodeVisitorInterface
+    interface NodeVisitorInterface extends \Twig_NodeVisitorInterface
     {
     }
 }
diff --git a/src/RuntimeLoader/RuntimeLoaderInterface.php b/src/RuntimeLoader/RuntimeLoaderInterface.php
index 255459413..4fadd455a 100644
--- a/src/RuntimeLoader/RuntimeLoaderInterface.php
+++ b/src/RuntimeLoader/RuntimeLoaderInterface.php
@@ -5,7 +5,7 @@ namespace Twig\RuntimeLoader;
 require __DIR__.'/../../lib/Twig/RuntimeLoaderInterface.php';
 
 if (\false) {
-    class RuntimeLoaderInterface extends \Twig_RuntimeLoaderInterface
+    interface RuntimeLoaderInterface extends \Twig_RuntimeLoaderInterface
     {
     }
 }
diff --git a/src/Sandbox/SecurityPolicyInterface.php b/src/Sandbox/SecurityPolicyInterface.php
index 1178998fd..b89ab958a 100644
--- a/src/Sandbox/SecurityPolicyInterface.php
+++ b/src/Sandbox/SecurityPolicyInterface.php
@@ -5,7 +5,7 @@ namespace Twig\Sandbox;
 require __DIR__.'/../../lib/Twig/Sandbox/SecurityPolicyInterface.php';
 
 if (\false) {
-    class SecurityPolicyInterface extends \Twig_Sandbox_SecurityPolicyInterface
+    interface SecurityPolicyInterface extends \Twig_Sandbox_SecurityPolicyInterface
     {
     }
 }
diff --git a/src/TokenParser/TokenParserInterface.php b/src/TokenParser/TokenParserInterface.php
index 334e10571..add6ed883 100644
--- a/src/TokenParser/TokenParserInterface.php
+++ b/src/TokenParser/TokenParserInterface.php
@@ -5,7 +5,7 @@ namespace Twig\TokenParser;
 require __DIR__.'/../../lib/Twig/TokenParserInterface.php';
 
 if (\false) {
-    class TokenParserInterface extends \Twig_TokenParserInterface
+    interface TokenParserInterface extends \Twig_TokenParserInterface
     {
     }
 }
diff --git a/src/Environment.php b/src/Twig.php
similarity index 67%
rename from src/Environment.php
rename to src/Twig.php
index a165f8b5c..c24bb1828 100644
--- a/src/Environment.php
+++ b/src/Twig.php
@@ -5,7 +5,7 @@ namespace Twig;
 require __DIR__.'/../lib/Twig/Environment.php';
 
 if (\false) {
-    class Environment extends \Twig_Environment
+    class Twig extends \Twig_Environment
     {
     }
 }
diff --git a/test/Twig/Tests/EnvironmentTest.php b/test/Twig/Tests/EnvironmentTest.php
index 2fde48a9d..5b07c40e8 100644
--- a/test/Twig/Tests/EnvironmentTest.php
+++ b/test/Twig/Tests/EnvironmentTest.php
@@ -311,6 +311,9 @@ class Twig_Tests_EnvironmentTest extends PHPUnit_Framework_TestCase
 
         $this->assertSame($ext, $twig->getExtension('Twig_Tests_EnvironmentTest_Extension'));
         $this->assertSame($ext, $twig->getExtension('\Twig_Tests_EnvironmentTest_Extension'));
+
+        $this->assertTrue($twig->hasExtension('Twig\Tests\EnvironmentTest\Extension'));
+        $this->assertSame($ext, $twig->getExtension('Twig\Tests\EnvironmentTest\Extension'));
     }
 
     public function testAddExtension()
@@ -557,6 +560,7 @@ class Twig_Tests_EnvironmentTest_Extension extends Twig_Extension implements Twi
         );
     }
 }
+class_alias('Twig_Tests_EnvironmentTest_Extension', 'Twig\Tests\EnvironmentTest\Extension', false);
 
 class Twig_Tests_EnvironmentTest_Extension_WithDeprecatedName extends Twig_Extension
 {