trying to fix #4113

Router.php.diff

diff --git a/classes/Kohana/Route.php b/classes/Kohana/Route.php
index fdd2e1b..d8e33fe 100644
--- a/classes/Kohana/Route.php
+++ b/classes/Kohana/Route.php
@@ -531,8 +531,12 @@ class Kohana_Route {
                        return rtrim($params['host'], '/').'/'.$uri;
                }
 
+               // Keep track of whether an optional param was replaced
+               $provided_optional = FALSE;
+
                while (preg_match('#\([^()]++\)#', $uri, $match))
                {
+
                        // Search for the matched value
                        $search = $match[0];
 
@@ -545,9 +549,27 @@ class Kohana_Route {
 
                                if (isset($params[$param]))
                                {
+                                       // Future optional params should be required
+                                       $provided_optional = TRUE;
+
                                        // Replace the key with the parameter value
                                        $replace = str_replace($key, $params[$param], $replace);
                                }
+                               elseif ($provided_optional)
+                               {
+                                       // Look for a default
+                                       if (isset($this->_defaults[$param]))
+                                       {
+                                               $replace = str_replace($key, $this->_defaults[$param], $replace);
+                                       }
+                                       else
+                                       {
+                                               // Ungrouped parameters are required
+                                               throw new Kohana_Exception('Required route parameter not passed: :param', array(
+                                                       ':param' => $param,
+                                               ));
+                                       }
+                               }
                                else
                                {
                                        // This group has missing parameters
@@ -577,7 +599,7 @@ class Kohana_Route {
                                        throw new Kohana_Exception('Required route parameter not passed: :param', array(
                                                ':param' => $param,
                                        ));
-                       }
+                               }
                        }
 
                        $uri = str_replace($key, $params[$param], $uri);