The debug class loader is always loaded by Debug::enable().

Debug.php

@@ -23,10 +23,7 @@ class Debug
     /**
      * Enables the debug tools.
      *
-     * This method registers an error handler and an exception handler.
-     *
-     * If the Symfony ClassLoader component is available, a special
-     * class loader is also registered.
+     * This method registers an error handler, an exception handler and a special class loader.
      *
      * @param int  $errorReportingLevel The level of error reporting you want
      * @param bool $displayErrors       Whether to display errors (for development) or just log them (for production)

Exception/ClassNotFoundException.php

@@ -26,6 +26,9 @@ class ClassNotFoundException extends FatalErrorException
             $previous->getSeverity(),
             $previous->getFile(),
             $previous->getLine(),
+            null,
+            true,
+            null,
             $previous->getPrevious()
         );
         $this->setTrace($previous->getTrace());

Exception/FatalErrorException.php

@@ -35,9 +35,9 @@ use Symfony\Component\HttpKernel\Exception\FatalErrorException as LegacyFatalErr
  */
 class FatalErrorException extends LegacyFatalErrorException
 {
-    public function __construct($message, $code, $severity, $filename, $lineno, $traceOffset = null, $traceArgs = true, array $trace = null)
+    public function __construct($message, $code, $severity, $filename, $lineno, $traceOffset = null, $traceArgs = true, array $trace = null, $previous = null)
     {
-        parent::__construct($message, $code, $severity, $filename, $lineno);
+        parent::__construct($message, $code, $severity, $filename, $lineno, $previous);
 
         if (null !== $trace) {
             if (!$traceArgs) {

Exception/UndefinedFunctionException.php

@@ -26,6 +26,9 @@ class UndefinedFunctionException extends FatalErrorException
             $previous->getSeverity(),
             $previous->getFile(),
             $previous->getLine(),
+            null,
+            true,
+            null,
             $previous->getPrevious()
         );
         $this->setTrace($previous->getTrace());

Exception/UndefinedMethodException.php

@@ -26,6 +26,9 @@ class UndefinedMethodException extends FatalErrorException
             $previous->getSeverity(),
             $previous->getFile(),
             $previous->getLine(),
+            null,
+            true,
+            null,
             $previous->getPrevious()
         );
         $this->setTrace($previous->getTrace());