diff --git a/src/ExtensionSet.php b/src/ExtensionSet.php
index 7cf7fa7ee..2bd40daf7 100644
--- a/src/ExtensionSet.php
+++ b/src/ExtensionSet.php
@@ -69,22 +69,12 @@ final class ExtensionSet
 
     public function hasExtension(string $class): bool
     {
-        $class = ltrim($class, '\\');
-        if (!isset($this->extensions[$class]) && class_exists($class, false)) {
-            // For BC/FC with namespaced aliases
-            $class = (new \ReflectionClass($class))->name;
-        }
-
-        return isset($this->extensions[$class]);
+        return isset($this->extensions[ltrim($class, '\\')]);
     }
 
     public function getExtension(string $class): ExtensionInterface
     {
         $class = ltrim($class, '\\');
-        if (!isset($this->extensions[$class]) && class_exists($class, false)) {
-            // For BC/FC with namespaced aliases
-            $class = (new \ReflectionClass($class))->name;
-        }
 
         if (!isset($this->extensions[$class])) {
             throw new RuntimeError(sprintf('The "%s" extension is not enabled.', $class));
@@ -149,8 +139,6 @@ final class ExtensionSet
             throw new \LogicException(sprintf('Unable to register extension "%s" as it is already registered.', $class));
         }
 
-        // For BC/FC with namespaced aliases
-        $class = (new \ReflectionClass($class))->name;
         $this->extensions[$class] = $extension;
     }
 
diff --git a/src/Node/Expression/CallExpression.php b/src/Node/Expression/CallExpression.php
index 455be50c1..8be84cab2 100644
--- a/src/Node/Expression/CallExpression.php
+++ b/src/Node/Expression/CallExpression.php
@@ -37,8 +37,7 @@ abstract class CallExpression extends AbstractExpression
                     $compiler->raw(sprintf('$this->env->getRuntime(\'%s\')->%s', $callable[0], $callable[1]));
                 }
             } elseif ($r instanceof \ReflectionMethod && $callable[0] instanceof ExtensionInterface) {
-                // For BC/FC with namespaced aliases
-                $class = (new \ReflectionClass(\get_class($callable[0])))->name;
+                $class = \get_class($callable[0]);
                 if (!$compiler->getEnvironment()->hasExtension($class)) {
                     // Compile a non-optimized call to trigger a \Twig\Error\RuntimeError, which cannot be a compile-time error
                     $compiler->raw(sprintf('$this->env->getExtension(\'%s\')', $class));