diff -u b/core/lib/Drupal/Core/Field/FormatterPluginManager.php b/core/lib/Drupal/Core/Field/FormatterPluginManager.php
--- b/core/lib/Drupal/Core/Field/FormatterPluginManager.php
+++ b/core/lib/Drupal/Core/Field/FormatterPluginManager.php
@@ -91,6 +91,8 @@
    *       also be used if the requested formatter is not available.
    *     - settings: (array) Settings specific to the formatter. Each setting
    *       defaults to the default value specified in the formatter definition.
+   *     - third_party_settings: (array) Settings provided by other extensions
+   *       through hook_field_formatter_third_party_settings_form().
    *
    * @return \Drupal\Core\Field\FormatterInterface|null
    *   A formatter object or NULL when plugin is not found.
diff -u b/core/lib/Drupal/Core/Field/PluginSettingsBase.php b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
--- b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
+++ b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
@@ -96,11 +96,8 @@
   /**
    * {@inheritdoc}
    */
-  public function getThirdPartySetting($module, $key) {
-    if (isset($this->thirdPartySettings[$module][$key])) {
-      return $this->thirdPartySettings[$module][$key];
-    }
-    return NULL;
+  public function getThirdPartySetting($module, $key, $default = NULL) {
+    return isset($this->thirdPartySettings[$module][$key]) ? $this->thirdPartySettings[$module][$key] : $default;
   }
 
   /**
diff -u b/core/lib/Drupal/Core/Field/PluginSettingsInterface.php b/core/lib/Drupal/Core/Field/PluginSettingsInterface.php
--- b/core/lib/Drupal/Core/Field/PluginSettingsInterface.php
+++ b/core/lib/Drupal/Core/Field/PluginSettingsInterface.php
@@ -65,17 +65,21 @@
   public function setSetting($key, $value);
 
   /**
-   * Returns the value of a third-party setting, or its default value if absent.
+   * Returns the value of a third-party setting, or $default if not set.
    *
    * @param string $module
    *   The module providing the third-party setting.
    * @param string $key
    *   The setting name.
+   * @param mixed $default
+   *   (optional) The default value if the third party setting is not set.
+   *   Defaults to NULL.
    *
    * @return mixed|NULL
-   *   The setting value. Returns NULL if the setting does not exist.
+   *   The setting value. Returns NULL if the setting does not exist and
+   *   $default is not provided.
    */
-  public function getThirdPartySetting($module, $key);
+  public function getThirdPartySetting($module, $key, $default = NULL);
 
   /**
    * Sets the value of a third-party setting for the plugin.
diff -u b/core/lib/Drupal/Core/Field/WidgetPluginManager.php b/core/lib/Drupal/Core/Field/WidgetPluginManager.php
--- b/core/lib/Drupal/Core/Field/WidgetPluginManager.php
+++ b/core/lib/Drupal/Core/Field/WidgetPluginManager.php
@@ -71,6 +71,8 @@
    *       used if the requested widget is not available.
    *     - settings: (array) Settings specific to the widget. Each setting
    *       defaults to the default value specified in the widget definition.
+   *     - third_party_settings: (array) Settings provided by other extensions
+   *       through hook_field_formatter_third_party_settings_form().
    *
    * @return \Drupal\Core\Field\WidgetInterface|null
    *   A Widget object or NULL when plugin is not found.
diff -u b/core/modules/field/tests/modules/field_third_party_test/field_third_party_test.module b/core/modules/field/tests/modules/field_third_party_test/field_third_party_test.module
--- b/core/modules/field/tests/modules/field_third_party_test/field_third_party_test.module
+++ b/core/modules/field/tests/modules/field_third_party_test/field_third_party_test.module
@@ -3,7 +3,7 @@
 /**
  * Implements hook_field_widget_third_party_settings_form().
  */
-function field_third_party_test_field_widget_third_party_settings_form($plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $form_mode, $form, $form_state) {
+function field_third_party_test_field_widget_third_party_settings_form(\Drupal\Core\Field\WidgetInterface $plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $form_mode, $form, $form_state) {
   $element['field_test_widget_third_party_settings_form'] = array(
     '#type' => 'textfield',
     '#title' => t('3rd party widget settings form'),
@@ -23,7 +23,7 @@
 /**
  * Implements hook_field_formatter_third_party_settings_form().
  */
-function field_third_party_test_field_formatter_third_party_settings_form($plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $view_mode, $form, $form_state) {
+function field_third_party_test_field_formatter_third_party_settings_form(\Drupal\Core\Field\FormatterInterface $plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $view_mode, $form, $form_state) {
   $element['field_test_field_formatter_third_party_settings_form'] = array(
     '#type' => 'textfield',
     '#title' => t('3rd party formatter settings form'),
diff -u b/core/modules/field_ui/field_ui.api.php b/core/modules/field_ui/field_ui.api.php
--- b/core/modules/field_ui/field_ui.api.php
+++ b/core/modules/field_ui/field_ui.api.php
@@ -13,14 +13,14 @@
 /**
  * Allow modules to add settings to field formatters provided by other modules.
  *
- * @param object $plugin
- *   The field formatter.
+ * @param \Drupal\Core\Field\FormatterInterface $plugin
+ *   The instantiated field formatter plugin.
  * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
  *   The field definition.
  * @param $view_mode
  *   The entity view mode.
  * @param array $form
- *   The The (entire) configuration form array.
+ *   The (entire) configuration form array.
  * @param array $form_state
  *   The form state.
  *
@@ -29,7 +29,7 @@
  *
  * @see \Drupal\field_ui\DisplayOverView.
  */
-function hook_field_formatter_third_party_settings_form($plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $view_mode, $form, $form_state) {
+function hook_field_formatter_third_party_settings_form(\Drupal\Core\Field\FormatterInterface $plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $view_mode, $form, $form_state) {
   $element = array();
   // Add a 'my_setting' checkbox to the settings form for 'foo_formatter' field
   // formatters.
@@ -46,14 +46,14 @@
 /**
  * Allow modules to add settings to field widgets provided by other modules.
  *
- * @param object $plugin
- *   The field widget.
+ * @param \Drupal\Core\Field\WidgetInterface $plugin
+ *   The instantiated field widget plugin.
  * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
  *   The field definition.
  * @param $form_mode
  *   The entity form mode.
  * @param array $form
- *   The The (entire) configuration form array.
+ *   The (entire) configuration form array.
  * @param array $form_state
  *   The form state.
  *
@@ -62,7 +62,7 @@
  *
  * @see \Drupal\field_ui\FormDisplayOverView.
  */
-function hook_field_widget_third_party_settings_form($plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $form_mode, $form, $form_state) {
+function hook_field_widget_third_party_settings_form(\Drupal\Core\Field\WidgetInterface $plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $form_mode, $form, $form_state) {
   $element = array();
   // Add a 'my_setting' checkbox to the settings form for 'foo_widget' field
   // widgets.
diff -u b/core/modules/field_ui/src/DisplayOverview.php b/core/modules/field_ui/src/DisplayOverview.php
--- b/core/modules/field_ui/src/DisplayOverview.php
+++ b/core/modules/field_ui/src/DisplayOverview.php
@@ -14,6 +14,8 @@
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Field\FieldTypePluginManager;
+use Drupal\Core\Field\FormatterInterface;
+use Drupal\Core\Field\PluginSettingsInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -230,11 +232,11 @@
   /**
    * {@inheritdoc}
    */
-  protected function alterSettingsForm(array &$settings_form, $plugin, FieldDefinitionInterface $field_definition, array $form, array &$form_state) {
-    // Iterate through the modules calling their
-    // field_formatter_third_party_settings_form handlers (if any):
+  protected function thirdPartySettingsForm(PluginSettingsInterface $plugin, FieldDefinitionInterface $field_definition, array $form, array &$form_state) {
+    $settings_form = array();
+    // Invoke hook_field_formatter_third_party_settings_form(), keying resulting
+    // subforms by module name.
     foreach ($this->moduleHandler->getImplementations('field_formatter_third_party_settings_form') as $module) {
-      // Index the settings form by module name.
       $settings_form[$module] = $this->moduleHandler->invoke($module, 'field_formatter_third_party_settings_form', array(
         $plugin,
         $field_definition,
@@ -243,12 +245,13 @@
         $form_state,
       ));
     }
+    return $settings_form;
   }
 
   /**
    * {@inheritdoc}
    */
-  protected function alterSettingsSummary(array &$summary, $plugin, FieldDefinitionInterface $field_definition) {
+  protected function alterSettingsSummary(array &$summary, PluginSettingsInterface $plugin, FieldDefinitionInterface $field_definition) {
     $context = array(
       'formatter' => $plugin,
       'field_definition' => $field_definition,
diff -u b/core/modules/field_ui/src/DisplayOverviewBase.php b/core/modules/field_ui/src/DisplayOverviewBase.php
--- b/core/modules/field_ui/src/DisplayOverviewBase.php
+++ b/core/modules/field_ui/src/DisplayOverviewBase.php
@@ -14,6 +14,7 @@
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\Core\Field\FieldTypePluginManagerInterface;
+use Drupal\Core\Field\PluginSettingsInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -352,8 +353,7 @@
       if ($plugin) {
         // Generate the settings form and allow other modules to alter it.
         $settings_form = $plugin->settingsForm($form, $form_state);
-        $third_party_settings_form = array();
-        $this->alterSettingsForm($third_party_settings_form, $plugin, $field_definition, $form, $form_state);
+        $third_party_settings_form = $this->thirdPartySettingsForm($plugin, $field_definition, $form, $form_state);
 
         if ($settings_form || $third_party_settings_form) {
           $field_row['plugin']['#cell_attributes'] = array('colspan' => 3);
@@ -411,8 +411,7 @@
 
         // Check selected plugin settings to display edit link or not.
         $settings_form = $plugin->settingsForm($form, $form_state);
-        $third_party_settings_form = array();
-        $this->alterSettingsForm($third_party_settings_form, $plugin, $field_definition, $form, $form_state);
+        $third_party_settings_form = $this->thirdPartySettingsForm($plugin, $field_definition, $form, $form_state);
         if (!empty($settings_form) || !empty($third_party_settings_form)) {
           $field_row['settings_edit'] = $base_button + array(
             '#type' => 'image_button',
@@ -874,30 +873,31 @@
 
   /**
-   * Alters the widget or formatter settings form.
+   * Adds the widget or formatter third party settings forms.
    *
-   * @param array $settings_form
-   *   The widget or formatter settings form.
-   * @param object $plugin
+   * @param \Drupal\Core\Field\PluginSettingsInterface $plugin
    *   The widget or formatter.
    * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
    *   The field definition.
    * @param array $form
-   *   The The (entire) configuration form array.
+   *   The (entire) configuration form array.
    * @param array $form_state
    *   The form state.
+   *
+   * @return array
+   *   The widget or formatter third party settings form.
    */
-  abstract protected function alterSettingsForm(array &$settings_form, $plugin, FieldDefinitionInterface $field_definition, array $form, array &$form_state);
+  abstract protected function thirdPartySettingsForm(PluginSettingsInterface $plugin, FieldDefinitionInterface $field_definition, array $form, array &$form_state);
 
   /**
    * Alters the widget or formatter settings summary.
    *
    * @param array $summary
    *   The widget or formatter settings summary.
-   * @param object $plugin
+   * @param \Drupal\Core\Field\PluginSettingsInterface $plugin
    *   The widget or formatter.
    * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
    *   The field definition.
    */
-  abstract protected function alterSettingsSummary(array &$summary, $plugin, FieldDefinitionInterface $field_definition);
+  abstract protected function alterSettingsSummary(array &$summary, PluginSettingsInterface $plugin, FieldDefinitionInterface $field_definition);
 
 }
diff -u b/core/modules/field_ui/src/FormDisplayOverview.php b/core/modules/field_ui/src/FormDisplayOverview.php
--- b/core/modules/field_ui/src/FormDisplayOverview.php
+++ b/core/modules/field_ui/src/FormDisplayOverview.php
@@ -14,6 +14,7 @@
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\Core\Field\FieldTypePluginManager;
+use Drupal\Core\Field\PluginSettingsInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -182,11 +183,11 @@
   /**
    * {@inheritdoc}
    */
-  protected function alterSettingsForm(array &$settings_form, $plugin, FieldDefinitionInterface $field_definition, array $form, array &$form_state) {
-    // Iterate through the modules calling their
-    // field_widget_third_party_settings_form handlers (if any):
+  protected function thirdPartySettingsForm(PluginSettingsInterface $plugin, FieldDefinitionInterface $field_definition, array $form, array &$form_state) {
+    $settings_form = array();
+    // Invoke hook_field_widget_third_party_settings_form(), keying resulting
+    // subforms by module name.
     foreach ($this->moduleHandler->getImplementations('field_widget_third_party_settings_form') as $module) {
-      // Index the settings form by module name.
       $settings_form[$module] = $this->moduleHandler->invoke($module, 'field_widget_third_party_settings_form', array(
         $plugin,
         $field_definition,
@@ -195,12 +196,13 @@
         $form_state,
       ));
     }
+    return $settings_form;
   }
 
   /**
    * {@inheritdoc}
    */
-  protected function alterSettingsSummary(array &$summary, $plugin, FieldDefinitionInterface $field_definition) {
+  protected function alterSettingsSummary(array &$summary, PluginSettingsInterface $plugin, FieldDefinitionInterface $field_definition) {
     $context = array(
       'widget' => $plugin,
       'field_definition' => $field_definition,
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php
+++ b/core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php
@@ -54,9 +54,8 @@ public function checkConfigSchema(TypedConfigManagerInterface $typed_config, $co
     }
     $definition = $typed_config->getDefinition($config_name);
     $this->schema = $typed_config->create($definition, $config_data);
-    $errors = array();
     foreach ($config_data as $key => $value) {
-      $errors = array_merge($errors, $this->checkValue($key, $value));
+      $errors = $this->checkValue($key, $value);
     }
     if (empty($errors)) {
       return TRUE;
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php
+++ b/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php
@@ -91,6 +91,8 @@ public function isDisplayConfigurable($display_context);
    *     for the field type will be used.
    *   - settings: (array) Settings for the plugin specified above. The default
    *     settings for the plugin will be used for settings left unspecified.
+   *   - third_party_settings: (array) Settings provided by other extensions
+   *     through hook_field_formatter_third_party_settings_form().
    *   - weight: (float) The weight of the element. Not needed if 'type' is
    *     'hidden'.
    *   The defaults of the various display options above get applied by the used