=== modified file 'includes/module.inc'
--- includes/module.inc	2007-11-30 09:16:02 +0000
+++ includes/module.inc	2007-12-08 15:52:56 +0000
@@ -147,6 +147,7 @@ function module_rebuild_cache() {
       db_query("INSERT INTO {system} (name, info, type, filename, status, throttle, bootstrap) VALUES ('%s', '%s', '%s', '%s', %d, %d, %d)", $file->name, serialize($files[$filename]->info), 'module', $file->filename, 0, 0, $bootstrap);
     }
   }
+  $files = _module_build_dependencies($files);
   $files = _module_build_dependents($files);
   return $files;
 }
@@ -174,6 +175,46 @@ function _module_build_dependents($files
   return $files;
 }
 
+function _module_build_dependencies($files) {
+  do {
+    $new_dependency = FALSE;
+    foreach ($files as $filename => $file) {
+      $file = &$files[$filename];
+      if (isset($file->info['dependencies']) && is_array($file->info['dependencies'])) {
+        foreach ($file->info['dependencies'] as $dependency_name) {
+          // This is a nonexistent module.
+          if ($dependency_name == '-circular-') {
+            continue;
+          }
+          $dependency = $files[$dependency_name];
+          if (isset($dependency->info['dependencies']) && is_array($dependency->info['dependencies'])) {
+            foreach ($dependency->info['dependencies'] as $candidate) {
+              if (array_search($candidate, $file->info['dependencies']) === FALSE) {
+                if ($candidate == $filename) {
+                  // As a module name can not contain dashes, this makes
+                  // impossible to switch on the module.
+                  $candidate = '-circular-';
+                  // Do not display the message or add -circular- more than once.
+                  if (array_search($candidate, $file->info['dependencies']) !== FALSE) {
+                    continue;
+                  }
+                  drupal_set_message(t('%module is part of a circular dependency. This is not supported and you will not be able to switch it on.', array('%module' => $file->info['name'])), 'error');
+                }
+                else {
+                  $new_dependency = TRUE;
+                }
+                $file->info['dependencies'][] = $candidate;
+              }
+            }
+          }
+        }
+      }
+      unset($file);
+    }
+  } while ($new_dependency);
+  return $files;
+}
+
 /**
  * Determine whether a given module exists.
  *
@@ -240,6 +281,23 @@ function module_load_all_includes($type,
  *   An array of module names.
  */
 function module_enable($module_list) {
+  $files = module_rebuild_cache();
+  do {
+    $moved = FALSE;
+    foreach ($module_list as $key => $module) {
+      $done[$module] = TRUE;
+      $file = $files[$module];
+      if (isset($file->info['dependencies']) && is_array($file->info['dependencies'])) {
+        foreach ($file->info['dependencies'] as $dependency) {
+          if (!isset($done[$dependency])) {
+            unset($module_list[$key]);
+            $module_list[] = $module;
+            $moved = TRUE;
+          }
+        }
+      }
+    }
+  } while ($moved);
   $invoke_modules = array();
   foreach ($module_list as $module) {
     $existing = db_fetch_object(db_query("SELECT status FROM {system} WHERE type = '%s' AND name = '%s'", 'module', $module));

=== modified file 'modules/system/system.admin.inc'
--- modules/system/system.admin.inc	2007-12-08 14:06:20 +0000
+++ modules/system/system.admin.inc	2007-12-08 15:23:50 +0000
@@ -535,6 +535,12 @@ function system_theme_settings_submit($f
  * Recursively check compatability
  */
 function _system_is_incompatible(&$incompatible, $files, $file) {
+  static $seen;
+  // We need to protect ourselves in case of a circular dependency.
+  if (isset($seen[$file->name])) {
+    return isset($incompatible[$file->name]);
+  }
+  $seen[$file->name] = TRUE;
   if (isset($incompatible[$file->name])) {
     return TRUE;
   }