diff --git a/core/lib/Drupal/Core/Render/Element/OptGroup.php b/core/lib/Drupal/Core/Render/Element/OptGroup.php
index a46e2a8..a2f7cab 100644
--- a/core/lib/Drupal/Core/Render/Element/OptGroup.php
+++ b/core/lib/Drupal/Core/Render/Element/OptGroup.php
@@ -62,40 +62,37 @@ public function getInfo() {
    * @see _form_validate()
    */
   public static function processOptGroup(&$element, FormStateInterface $form_state, &$complete_form) {
-    // @TODO This check is taken over from \Drupal\Core\Render\Element\Radios
-    //   and \Drupal\Core\Render\Element\Checkboxes, but I do not understand why
-    //   it is necessary.
-    if (count($element['#options']) > 0) {
-      // @TODO This is ported from form_select_options(). But instead of
-      //   checking #value, #default_value should be used here and below, right?
-      $value_valid = isset($element['#value']) || array_key_exists('#value', $element);
-      $value_is_array = $value_valid && is_array($element['#value']);
-      // Check if the element is multiple select and no value has been selected.
-      $empty_value = (empty($element['#value']) && !empty($element['#multiple']));
-
-      $weight = 0;
-      foreach ($element['#options'] as $key => $choice) {
-        $key = (string) $key;
-
-        // Maintain order of options as defined in #options, in case the element
-        // defines custom option sub-elements, but does not define all option
-        // sub-elements.
-        $weight += 0.001;
-
-        $empty_choice = $empty_value && $key == '_none';
-
-        $element += array($key => array());
-        $element[$key] += array(
-          '#type' => 'option',
-          '#title' => $choice,
-          // The key is sanitized in Drupal\Core\Template\Attribute during output
-          // from the theme function.
-          '#return_value' => $key,
-          '#selected' => $value_valid && ((!$value_is_array && (string) $element['#value'] === $key || ($value_is_array && in_array($key, $element['#value']))) || $empty_choice),
-          '#ajax' => isset($element['#ajax']) ? $element['#ajax'] : NULL,
-          '#weight' => $weight,
-        );
-      }
+    // @TODO This is ported from form_select_options(). But instead of
+    //   checking #value, #default_value should be used here and below, right?
+    $value_valid = isset($element['#value']) || array_key_exists('#value', $element);
+    $value_is_array = $value_valid && is_array($element['#value']);
+    // Check if the element is multiple select and no value has been selected.
+    $empty_value = (empty($element['#value']) && !empty($element['#multiple']));
+
+    $weight = 0;
+    foreach ($element['#options'] as $original_key => $choice) {
+      $key = (string) $original_key;
+
+      // Maintain order of options as defined in #options, in case the element
+      // defines custom option sub-elements, but does not define all option
+      // sub-elements.
+      $weight += 0.001;
+
+      $empty_choice = $empty_value && $key == '_none';
+
+      $element += array($key => array());
+      $element[$key] += array(
+        '#type' => 'option',
+        '#title' => $choice,
+        // The key is sanitized in Drupal\Core\Template\Attribute during output
+        // from the theme function.
+        '#return_value' => $key,
+        '#selected' => $value_valid && ((!$value_is_array && (string) $element['#value'] === $key || ($value_is_array && in_array($key, $element['#value']))) || $empty_choice),
+        '#ajax' => isset($element['#ajax']) ? $element['#ajax'] : NULL,
+        '#weight' => $weight,
+      );
+
+      unset($element['#options'][$original_key]);
     }
 
     return $element;
@@ -118,8 +115,7 @@ public static function preRenderOptGroup($element) {
   public static function getOptions($element) {
     $options = [];
 
-    // @TODO This does not respect weights.
-    foreach (Element::children($element) as $child) {
+    foreach (Element::children($element, TRUE) as $child) {
       if ($element[$child]['#type'] == 'option') {
         $options[$child] = $element[$child]['#title'];
       }
diff --git a/core/lib/Drupal/Core/Render/Element/Option.php b/core/lib/Drupal/Core/Render/Element/Option.php
index 036a10e..0c6b6b6 100644
--- a/core/lib/Drupal/Core/Render/Element/Option.php
+++ b/core/lib/Drupal/Core/Render/Element/Option.php
@@ -33,6 +33,7 @@ public function getInfo() {
       ),
       '#theme' => 'option',
       '#selected' => FALSE,
+      '#return_value' => NULL,
     );
   }
 
diff --git a/core/lib/Drupal/Core/Render/Element/Select.php b/core/lib/Drupal/Core/Render/Element/Select.php
index ad3f13d..91fd942 100644
--- a/core/lib/Drupal/Core/Render/Element/Select.php
+++ b/core/lib/Drupal/Core/Render/Element/Select.php
@@ -129,8 +129,8 @@ public static function processSelect(&$element, FormStateInterface $form_state,
 
     // Process options and option groups.
     $weight = 0;
-    foreach ($element['#options'] as $key => $choice) {
-      $key = (string) $key;
+    foreach ($element['#options'] as $original_key => $choice) {
+      $key = (string) $original_key;
 
       // Maintain order of options as defined in #options, in case both
       // options and option groups are defined or the element defines custom
@@ -152,12 +152,13 @@ public static function processSelect(&$element, FormStateInterface $form_state,
         );
         OptGroup::processOptGroup($element[$key], $form_state, $complete_form);
 
-        unset($element['#options'][$key]);
+        unset($element['#options'][$original_key]);
       }
     }
 
     // Process the '#options' element in the same way that option groups do.
     OptGroup::processOptGroup($element, $form_state, $complete_form);
+    // At this point '#options' should be an empty array.
 
     return $element;
   }
@@ -211,14 +212,14 @@ public static function preRenderSelect($element) {
   public static function getFlattenedOptions($element) {
     $options = [];
 
-    // @TODO This does not respect weights.
-    foreach (Element::children($element) as $child) {
+    foreach (Element::children($element, TRUE) as $child) {
       if ($element[$child]['#type'] == 'optgroup') {
         $options += OptGroup::getOptions($element[$child]);
-        unset($element[$child]);
+      }
+      elseif ($element[$child]['#type'] == 'option') {
+        $options[$child] = $element[$child]['#title'];
       }
     }
-    $options += OptGroup::getOptions($element);
 
     return $options;
   }