diff --git a/core/lib/Drupal/Core/Form/FormValidator.php b/core/lib/Drupal/Core/Form/FormValidator.php
index ee2da65..51dd3fc 100644
--- a/core/lib/Drupal/Core/Form/FormValidator.php
+++ b/core/lib/Drupal/Core/Form/FormValidator.php
@@ -316,7 +316,7 @@ protected function performRequiredValidation(&$elements, FormStateInterface &$fo
 
     if (isset($elements['#options']) && isset($elements['#value'])) {
       if ($elements['#type'] == 'select') {
-        $options = OptGroup::flattenOptions($elements['#options']);
+        $options = Element\Select::getFlattenedOptions($elements);
       }
       else {
         $options = $elements['#options'];
diff --git a/core/lib/Drupal/Core/Render/Element/Optgroup.php b/core/lib/Drupal/Core/Render/Element/Optgroup.php
index 04b963c..12290c7 100644
--- a/core/lib/Drupal/Core/Render/Element/Optgroup.php
+++ b/core/lib/Drupal/Core/Render/Element/Optgroup.php
@@ -109,4 +109,23 @@ public static function preRenderOptgroup($element) {
     return $element;
   }
 
+  /**
+   * Returns a list of options for an option group element.
+   *
+   * This function assumes that the passed element has been processed by
+   * Optgroup::processOptgroup() prior to calling this function.
+   */
+  public static function getOptions($element) {
+    $options = [];
+
+    // @TODO This does not respect weights.
+    foreach (Element::children($element) as $child) {
+      if ($element[$child]['#type'] == 'option') {
+        $options[$child] = $element[$child]['#title'];
+      }
+    }
+
+    return $options;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Render/Element/Select.php b/core/lib/Drupal/Core/Render/Element/Select.php
index beae6bc..c4ab21d 100644
--- a/core/lib/Drupal/Core/Render/Element/Select.php
+++ b/core/lib/Drupal/Core/Render/Element/Select.php
@@ -212,4 +212,25 @@ public static function preRenderSelect($element) {
     return $element;
   }
 
+  /**
+   * Returns a list of flattened options for a select element.
+   *
+   * This function assumes that the passed element has been processed by
+   * Select::processSelect() prior to calling this function.
+   */
+  public static function getFlattenedOptions($element) {
+    $options = [];
+
+    // @TODO This does not respect weights.
+    foreach (Element::children($element) as $child) {
+      if ($element[$child]['#type'] == 'optgroup') {
+        $options += Optgroup::getOptions($element[$child]);
+        unset($element[$child]);
+      }
+    }
+    $options += Optgroup::getOptions($element);
+
+    return $options;
+  }
+
 }