diff --git a/core/default.settings.php b/core/default.settings.php
index 0181dbc..23ec5bc 100644
--- a/core/default.settings.php
+++ b/core/default.settings.php
@@ -45,8 +45,6 @@
  * hostname with that number. For example,
  * http://www.drupal.org:8080/mysite/test/ could be loaded from
  * sites/8080.www.drupal.org.mysite.test/.
- *
- * @see conf_path()
  */
 
 /**
@@ -66,16 +64,16 @@
  * the database (files, system table, etc.) this will ensure the paths are
  * correct when the site is deployed to a live server.
  *
- * Aliases are defined in an associative array in the format:
+ * Aliases are defined in an associative array named $sites. The array is
+ * written in the format: '<port>.<domain>.<path>' => 'directory'. As an
+ * example, to map http://www.drupal.org:8080/mysite/test to the configuration
+ * directory sites/example.com, the array should be defined as:
  * @code
- *   '<port>.<domain>.<path>' => 'directory',
- * @endcode
- * For example, to map http://www.drupal.org:8080/mysite/test to the directory
- * sites/example.com, the array should be defined as:
- * @code
- * $sites['8080.www.drupal.org.mysite.test'] = 'example.com';
+ * $sites = array(
+ *   '8080.www.drupal.org.mysite.test' => 'example.com',
+ * );
  * @endcode
- * The URL http://www.drupal.org:8080/mysite/test/ could be a symbolic link or
+ * The URL, http://www.drupal.org:8080/mysite/test/, could be a symbolic link or
  * an Apache Alias directive that points to the Drupal root containing
  * index.php. An alias could also be created for a subdomain. See the
  * @link http://drupal.org/documentation/install online Drupal installation guide @endlink
@@ -83,16 +81,21 @@
  *
  * The following examples look for a site configuration in sites/example.com:
  * @code
- * # URL: http://dev.drupal.org
+ * URL: http://dev.drupal.org
  * $sites['dev.drupal.org'] = 'example.com';
- * # URL: http://localhost/example
+ *
+ * URL: http://localhost/example
  * $sites['localhost.example'] = 'example.com';
- * # URL: http://localhost:8080/example
+ *
+ * URL: http://localhost:8080/example
  * $sites['8080.localhost.example'] = 'example.com';
- * # URL: http://www.drupal.org:8080/mysite/test/
+ *
+ * URL: http://www.drupal.org:8080/mysite/test/
  * $sites['8080.www.drupal.org.mysite.test'] = 'example.com';
  * @endcode
  *
+ * @see default.settings.php
+ * @see conf_path()
  * @see http://drupal.org/documentation/install/multi-site
  */
 # $sites = array();
diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 98f8c6a..f41807b 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -256,6 +256,11 @@ function timer_stop($name) {
  *
  * @param string $custom_path
  *   (optional) A custom $conf_path to set and statically cache.
+ * @param bool $require_settings
+ *   Only configuration directories with an existing settings.php file
+ *   will be recognized. Defaults to TRUE. During initial installation,
+ *   this is set to FALSE so that Drupal can detect a matching directory,
+ *   then create a new settings.php file in it.
  * @param bool $reset
  *   Force a full search for matching directories even if one had been
  *   found previously. Defaults to FALSE.
@@ -265,11 +270,18 @@ function timer_stop($name) {
  *
  * @see default.settings.php
  */
-function conf_path($custom_path = NULL, $reset = FALSE) {
+function conf_path($custom_path = NULL, $require_settings = TRUE, $reset = FALSE) {
   static $conf_path;
   global $sites;
 
-  if (!empty($custom_path) && is_string($custom_path)) {
+  if (isset($conf_path) && !$reset) {
+    return $conf_path;
+  }
+
+  if (isset($custom_path)) {
+    if (!is_string($custom_path)) {
+      throw new \InvalidArgumentException('The first argument to conf_path() must be a string.');
+    }
     $conf_path = $custom_path;
     return $conf_path;
   }
@@ -291,19 +303,12 @@ function conf_path($custom_path = NULL, $reset = FALSE) {
     if (!$script_name) {
       $script_name = $_SERVER['SCRIPT_FILENAME'];
     }
-    $require_settings = (bool) $custom_path;
     $conf_path = find_conf_path($http_host, $script_name, $require_settings, $sites);
   }
   // If the multi-site functionality is not enabled, $conf_path is empty, which
   // means that the root/top-level directory is the site directory.
   else {
-    // Many callers of this function append a slash to the returned path and do
-    // not prefix it with DRUPAL_ROOT, so Drupal writes new files and
-    // directories to the filesystem's root directory (which is possible on
-    // unsecured local Windows development environments). We therefore need to
-    // return a dot instead of an empty string to ensure that the path is
-    // interpreted as a relative path.
-    $conf_path = '.';
+    $conf_path = '';
   }
   return $conf_path;
 }
@@ -411,6 +416,17 @@ function find_conf_path($http_host, $script_name, $require_settings = TRUE, arra
   return 'sites/default';
 }
 
+function site_path($file_path, $absolute = FALSE) {
+  $conf_path = conf_path();
+  if ($file_path[0] === '/') {
+    $file_path = substr($file_path, 1);
+  }
+  if ($conf_path !== '') {
+    $file_path = $conf_path . '/' . $file_path;
+  }
+  return $absolute ? DRUPAL_ROOT . '/' . $file_path : $file_path;
+}
+
 /**
  * Returns the path of a configuration directory.
  *
@@ -584,25 +600,24 @@ function drupal_settings_initialize($require_settings = TRUE) {
   // Export these settings.php variables to the global namespace.
   global $sites, $databases, $cookie_domain, $conf, $db_prefix, $drupal_hash_salt, $base_secure_url, $base_insecure_url, $config_directories;
   $conf = array();
-  // Include the global settings.php to determine whether the multi-site
-  // functionality is enabled. If settings.php is not required, then the caller
-  // is likely the installer, in which case the global settings.php may be used
-  // to enable the multi-site functionality to install Drupal into a
-  // site-specific directory.
+
+  // Include settings.php to determine whether multi-site functionality is
+  // enabled. In the installer, $require_settings is FALSE, in which case the
+  // global settings.php may be used to enable the multi-site functionality
+  // to install Drupal into a site-specific directory.
   if ($require_settings || is_readable(DRUPAL_ROOT . '/settings.php')) {
     require_once DRUPAL_ROOT . '/settings.php';
   }
 
   // Discover the site directory, priming the static cache in conf_path().
   // Make conf_path() available as local variable in settings.php.
-  $conf_path = conf_path(isset($conf_path) ? $conf_path : $require_settings);
+  $conf_path = conf_path(isset($conf_path) ? $conf_path : NULL, $require_settings);
   if (isset($sites) && $conf_path !== '.' && is_readable(DRUPAL_ROOT . '/' . $conf_path . '/settings.php')) {
     include_once DRUPAL_ROOT . '/' . $conf_path . '/settings.php';
   }
   require_once __DIR__ . '../../lib/Drupal/Component/Utility/Settings.php';
 
   new Settings(isset($settings) ? $settings : array());
-
   $is_https = isset($_SERVER['HTTPS']) && strtolower($_SERVER['HTTPS']) == 'on';
 
   if (isset($base_url)) {
@@ -1996,11 +2011,7 @@ function _drupal_bootstrap_configuration() {
   // Load the procedural configuration system helper functions.
   require_once __DIR__ . '/config.inc';
 
-  // Set the Drupal custom error handler.
-  // Circular dependency: The error/exception handler requires config, which
-  // requires the service container, which requires the class loader, which
-  // requires $conf/settings.php (to be swappable). Therefore, errors in early
-  // bootstrap and settings.php cannot be handled by Drupal.
+  // Set the Drupal custom error handler. (requires \Drupal::config())
   set_error_handler('_drupal_error_handler');
   set_exception_handler('_drupal_exception_handler');
 
@@ -2300,12 +2311,13 @@ function _drupal_load_test_overrides($test_prefix) {
   }
 
   // Check for and load a settings.php file in the simpletest files directory.
-  $filename = conf_path() . '/files/' . $path_prefix . '/settings.php';
+  $filename = DRUPAL_ROOT . '/' . conf_path() . '/files/' . $path_prefix . '/settings.php';
   if (file_exists($filename)) {
     $settings = settings()->getAll();
-    $conf_path = &drupal_static('conf_path');
+    $conf_path = conf_path();
     // This can override $conf, $conf_path, $settings, and $config_directories.
     include $filename;
+    conf_path($conf_path);
     // Keep the overriden $conf_path alive across drupal_static_reset() calls.
     // @see conf_path()
     $settings['simpletest_conf_path'] = $conf_path;
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index a872e20..7a8f650 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -284,11 +284,8 @@ function install_begin_request(&$install_state) {
   }
 
   // Perform a minimal bootstrap.
-  // This essentially calls _drupal_bootstrap_configuration(), which in turn
-  // calls drupal_settings_initialize() using the return value of
-  // drupal_installation_attempted() as argument. During installation, this
-  // causes drupal_settings_initialize() to not attempt to load any settings.php
-  // files, which allows to prime the site path to be used during installation.
+  // During installation, drupal_settings_initialize() will not attempt to load
+  // settings.php, so as to prime the site path to be used during installation.
   // By not requiring settings.php, a bare site folder can be prepared in the
   // /sites directory, which will be used for installing Drupal.
   drupal_bootstrap(DRUPAL_BOOTSTRAP_CONFIGURATION);
@@ -1082,8 +1079,7 @@ function install_verify_database_settings() {
   global $databases;
   if (!empty($databases)) {
     $database = $databases['default']['default'];
-    $settings_file = './' . conf_path(FALSE) . '/settings.php';
-    $errors = install_database_errors($database, $settings_file);
+    $errors = install_database_errors($database);
     if (empty($errors)) {
       return TRUE;
     }
@@ -1104,8 +1100,6 @@ function install_verify_database_settings() {
 function install_settings_form($form, &$form_state, &$install_state) {
   global $databases;
 
-  $conf_path = './' . conf_path(FALSE);
-  $settings_file = $conf_path . '/settings.php';
   $database = isset($databases['default']['default']) ? $databases['default']['default'] : array();
 
   drupal_set_title(t('Database configuration'));
@@ -1152,7 +1146,6 @@ function install_settings_form($form, &$form_state, &$install_state) {
   );
 
   $form['errors'] = array();
-  $form['settings_file'] = array('#type' => 'value', '#value' => $settings_file);
 
   return $form;
 }
@@ -1186,7 +1179,7 @@ function install_settings_form_validate($form, &$form_state) {
   unset($database['db_prefix']);
 
   $form_state['storage']['database'] = $database;
-  $errors = install_database_errors($database, $form_state['values']['settings_file']);
+  $errors = install_database_errors($database);
   foreach ($errors as $name => $message) {
     form_set_error($name, $form_state, $message);
   }
@@ -1195,7 +1188,7 @@ function install_settings_form_validate($form, &$form_state) {
 /**
  * Checks a database connection and returns any errors.
  */
-function install_database_errors($database, $settings_file) {
+function install_database_errors($database) {
   global $databases;
   $errors = array();
 
@@ -1203,7 +1196,11 @@ function install_database_errors($database, $settings_file) {
   $database_types = drupal_get_database_types();
   $driver = $database['driver'];
   if (!isset($database_types[$driver])) {
-    $errors['driver'] = t("In your %settings_file file you have configured @drupal to use a %driver server, however your PHP installation currently does not support this database type.", array('%settings_file' => $settings_file, '@drupal' => drupal_install_profile_distribution_name(), '%driver' => $driver));
+    $errors['driver'] = t("In your %settings_file file you have configured @drupal to use a %driver server, however your PHP installation currently does not support this database type.", array(
+      '%settings_file' => site_path('settings.php'),
+      '@drupal' => drupal_install_profile_distribution_name(),
+      '%driver' => $driver,
+    ));
   }
   else {
     // Run driver specific validation
@@ -1501,7 +1498,7 @@ function install_translations_directory() {
     $directory = $GLOBALS['conf']['locale.settings']['translation.path'];
   }
   else {
-    $directory = conf_path(FALSE) . '/files/translations';
+    $directory = site_path('files/translations');
   }
   return $directory;
 }
@@ -2084,7 +2081,7 @@ function install_configure_form($form, &$form_state, &$install_state) {
 
   // Warn about settings.php permissions risk
   $settings_dir = conf_path();
-  $settings_file = $settings_dir . '/settings.php';
+  $settings_file = site_path('settings.php');
   // Check that $_POST is empty so we only show this message when the form is
   // first displayed, not on the next page after it is submitted. (We do not
   // want to repeat it multiple times because it is a general warning that is
@@ -2093,7 +2090,7 @@ function install_configure_form($form, &$form_state, &$install_state) {
   // distract from the message that the Drupal installation has completed
   // successfully.)
   $post_params = \Drupal::request()->request->all();
-  if (empty($post_params) && (!drupal_verify_install_file(DRUPAL_ROOT . '/' . $settings_file, FILE_EXIST|FILE_READABLE|FILE_NOT_WRITABLE) || !drupal_verify_install_file(DRUPAL_ROOT . '/' . $settings_dir, FILE_NOT_WRITABLE, 'dir'))) {
+  if (empty($post_params) && (!drupal_verify_install_file(site_path('settings.php', TRUE), FILE_EXIST|FILE_READABLE|FILE_NOT_WRITABLE) || !drupal_verify_install_file(site_path('', TRUE), FILE_NOT_WRITABLE, 'dir'))) {
     drupal_set_message(t('All necessary changes to %dir and %file have been made, so you should remove write permissions to them now in order to avoid security risks. If you are unsure how to do so, consult the <a href="@handbook_url">online handbook</a>.', array('%dir' => $settings_dir, '%file' => $settings_file, '@handbook_url' => 'http://drupal.org/server-permissions')), 'warning');
   }
 
@@ -2239,8 +2236,8 @@ function install_check_translations($install_state) {
   $readable = FALSE;
   $writable = FALSE;
   // @todo: Make this configurable.
-  $files_directory = conf_path(FALSE) . '/files';
-  $translations_directory = conf_path(FALSE) . '/files/translations';
+  $files_directory = site_path('files');
+  $translations_directory = site_path('files/translations');
   $translations_directory_exists = FALSE;
   $translation_available = FALSE;
   $online = FALSE;
@@ -2393,8 +2390,8 @@ function install_check_requirements($install_state) {
   if (!$install_state['settings_verified']) {
     $readable = FALSE;
     $writable = FALSE;
-    $conf_path = './' . conf_path(FALSE);
-    $settings_file = $conf_path . '/settings.php';
+    $conf_path = site_path('', TRUE);
+    $settings_file = site_path('settings.php');
     $default_settings_file = './core/default.settings.php';
     $file = $conf_path;
     $exists = FALSE;
diff --git a/core/includes/install.inc b/core/includes/install.inc
index 256c79e..35d245b 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -195,7 +195,7 @@ function drupal_get_database_types() {
  */
 function drupal_rewrite_settings($settings = array(), $settings_file = NULL) {
   if (!isset($settings_file)) {
-    $settings_file = conf_path(FALSE) . '/settings.php';
+    $settings_file = site_path('settings.php');
   }
   // Build list of setting names and insert the values into the global namespace.
   $variable_names = array();
@@ -449,11 +449,11 @@ function drupal_install_config_directories($mode = NULL) {
     $config_directories_hash = Crypt::randomStringHashed(55);
     $settings['config_directories'] = array(
       CONFIG_ACTIVE_DIRECTORY => (object) array(
-        'value' => conf_path() . '/files/config_' . $config_directories_hash . '/active',
+        'value' => site_path('files/config_' . $config_directories_hash . '/active'),
         'required' => TRUE,
       ),
       CONFIG_STAGING_DIRECTORY => (object) array(
-        'value' => conf_path() . '/files/config_' . $config_directories_hash . '/staging',
+        'value' => site_path('files/config_' . $config_directories_hash . '/staging'),
         'required' => TRUE,
       ),
     );
diff --git a/core/includes/update.inc b/core/includes/update.inc
index 24f6583..7f97de0 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -124,7 +124,7 @@ function update_prepare_d8_bootstrap() {
   // If any of the required settings needs to be written, then settings.php
   // needs to be writable.
   if (!$settings_exist) {
-    $settings_file = conf_path() . '/settings.php';
+    $settings_file = site_path('settings.php');
     $writable = drupal_verify_install_file($settings_file, FILE_EXIST | FILE_READABLE | FILE_WRITABLE);
     $requirements['settings file']['title'] = 'Settings file';
     if ($writable) {
diff --git a/core/lib/Drupal/Core/Database/Driver/sqlite/Install/Tasks.php b/core/lib/Drupal/Core/Database/Driver/sqlite/Install/Tasks.php
index 633edc6..8b972f2 100644
--- a/core/lib/Drupal/Core/Database/Driver/sqlite/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Driver/sqlite/Install/Tasks.php
@@ -49,7 +49,7 @@ public function getFormOptions(array $database) {
     // Make the text more accurate for SQLite.
     $form['database']['#title'] = t('Database file');
     $form['database']['#description'] = t('The absolute path to the file where @drupal data will be stored. This must be writable by the web server and should exist outside of the web root.', array('@drupal' => drupal_install_profile_distribution_name()));
-    $default_database = conf_path(FALSE, TRUE) . '/files/.ht.sqlite';
+    $default_database = site_path('files/.ht.sqlite');
     $form['database']['#default_value'] = empty($database['database']) ? $default_database : $database['database'];
     return $form;
   }
diff --git a/core/lib/Drupal/Core/StreamWrapper/PublicStream.php b/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
index 90f17b3..6fb15a8 100644
--- a/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
+++ b/core/lib/Drupal/Core/StreamWrapper/PublicStream.php
@@ -37,7 +37,7 @@ public function getExternalUrl() {
    *   The base path for public:// typically sites/default/files.
    */
   public static function basePath() {
-    $base_path = settings()->get('file_public_path', conf_path() . '/files');
+    $base_path = settings()->get('file_public_path', site_path('files'));
     if ($test_prefix = drupal_valid_test_ua()) {
       // Append the testing suffix unless already given.
       // @see \Drupal\simpletest\WebTestBase::setUp()
diff --git a/core/lib/Drupal/Core/SystemListing.php b/core/lib/Drupal/Core/SystemListing.php
index 20d95cb..676b994 100644
--- a/core/lib/Drupal/Core/SystemListing.php
+++ b/core/lib/Drupal/Core/SystemListing.php
@@ -85,7 +85,6 @@ function scan($mask, $directory, $key = 'name') {
     if (!in_array($key, array('uri', 'filename', 'name'))) {
       $key = 'uri';
     }
-    $config = conf_path();
 
     // Search for the directory in core.
     $searchdir = array('core/' . $directory);
@@ -100,9 +99,13 @@ function scan($mask, $directory, $key = 'name') {
     $searchdir[] = $directory;
     $searchdir[] = 'sites/all/' . $directory;
 
-    if (file_exists("$config/$directory")) {
-      $searchdir[] = "$config/$directory";
+    if (conf_path() !== '') {
+      $site_path = site_path($directory);
+      if (file_exists($site_path)) {
+        $searchdir[] = $site_path;
+      }
     }
+
     // @todo Find a way to skip ./config directories (but not modules/config).
     $nomask = '/^(CVS|lib|templates|css|js)$/';
     $files = array();
diff --git a/core/modules/locale/locale.install b/core/modules/locale/locale.install
index 960381c..a3e494e 100644
--- a/core/modules/locale/locale.install
+++ b/core/modules/locale/locale.install
@@ -13,7 +13,7 @@
 function locale_install() {
   // Create the interface translations directory and ensure it's writable.
   if (!$directory = \Drupal::config('locale.settings')->get('translation.path')) {
-    $directory = conf_path() . '/files/translations';
+    $directory = site_path('files/translations');
     \Drupal::config('locale.settings')->set('translation.path', $directory)->save();
   }
   file_prepare_directory($directory, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS);
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 560afb1..d90b32a 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -925,7 +925,7 @@ protected function prepareEnvironment() {
     // Save further contextual information.
     // Use the original files directory to avoid nesting it within an existing
     // simpletest directory if a test is executed within a test.
-    $this->originalFileDirectory = settings()->get('file_public_path', conf_path() . '/files');
+    $this->originalFileDirectory = settings()->get('file_public_path', site_path('files'));
     $this->originalProfile = drupal_get_profile();
     $this->originalUser = isset($user) ? clone $user : NULL;
 
@@ -1013,7 +1013,7 @@ protected function prepareConfigDirectories() {
     include_once DRUPAL_ROOT . '/core/includes/install.inc';
     foreach (array(CONFIG_ACTIVE_DIRECTORY, CONFIG_STAGING_DIRECTORY) as $type) {
       // Assign the relative path to the global variable.
-      $path = conf_path() . '/files/simpletest/' . substr($this->databasePrefix, 10) . '/config_' . $type;
+      $path = site_path('files/simpletest/' . substr($this->databasePrefix, 10) . '/config_' . $type);
       $GLOBALS['config_directories'][$type] = $path;
       // Ensure the directory can be created and is writeable.
       if (!install_ensure_config_directory($type)) {