commit 1aa8b6ed9c3e96d07a71fc2d22c4db77f18ae8e9
Author: Joel Pittet <joel@pittet.ca>
Date:   Fri Jul 31 22:32:46 2015 -0700

    no need for value callback

diff --git a/core/lib/Drupal/Core/Render/Element/Radio.php b/core/lib/Drupal/Core/Render/Element/Radio.php
index 5a7996f..022b5c2 100644
--- a/core/lib/Drupal/Core/Render/Element/Radio.php
+++ b/core/lib/Drupal/Core/Render/Element/Radio.php
@@ -45,38 +45,6 @@ public function getInfo() {
   }
 
   /**
-   * {@inheritdoc}
-   */
-  public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
-    if ($input === FALSE) {
-      // Use #default_value as the default value of a radio, except change
-      // NULL to 0, because FormBuilder::handleInputElement() would otherwise
-      // replace NULL with empty string, but an empty string is a potentially
-      // valid value for a checked radio.
-      return isset($element['#default_value']) ? $element['#default_value'] : 0;
-    }
-    else {
-      // Checked radios are submitted with a value (possibly '0' or ''):
-      // http://www.w3.org/TR/html401/interact/forms.html#successful-controls.
-      // For checked radios, browsers submit the string version of
-      // #return_value, but we return the original #return_value. For unchecked
-      // radios, browsers submit nothing at all, but
-      // FormBuilder::handleInputElement() detects this, and calls this
-      // function with $input=NULL. Returning NULL from a value callback means
-      // to use the default value, which is not what is wanted when an unchecked
-      // radio is submitted, so we use integer 0 as the value indicating an
-      // unchecked radio. Therefore, modules must not use integer 0 as a
-      // #return_value, as doing so results in the radio always being treated
-      // as unchecked. The string '0' is allowed for #return_value. The most
-      // common use-case for setting #return_value to either 0 or '0' is for the
-      // first option within a 0-indexed array of radios, and for this,
-      // \Drupal\Core\Render\Element\Checkboxes::processCheckboxes() uses the
-      // string rather than the integer.
-      return isset($input) ? $element['#return_value'] : 0;
-    }
-  }
-
-  /**
    * Prepares a #type 'radio' render element for input.html.twig.
    *
    * @param array $element
@@ -95,7 +63,7 @@ public static function preRenderRadio($element) {
     $element['#attributes']['type'] = 'radio';
     Element::setAttributes($element, array('id', 'name', '#return_value' => 'value'));
 
-    // Unchecked radio has #value of integer 0.
+    // Unchecked radio has #value of FALSE.
     if (!empty($element['#checked'])) {
       $element['#attributes']['checked'] = 'checked';
     }
@@ -112,9 +80,9 @@ public static function processRadio(&$element, FormStateInterface $form_state, &
     $return_value = $element['#return_value'];
     // On form submission, the #value of an available and enabled checked
     // radio is #return_value, and the #value of an available and enabled
-    // unchecked radio is integer 0. On not submitted forms, and for
+    // unchecked radio is FALSE. On not submitted forms, and for
     // radios with #access=FALSE or #disabled=TRUE, the #value is
-    // #default_value (integer 0 if #default_value is NULL). Most of the time,
+    // #default_value (FALSE if #default_value is NULL). Most of the time,
     // a string comparison of #value and #return_value is sufficient for
     // determining the "checked" state, but a value of TRUE always means checked
     // (even if #return_value is 'foo'), and a value of FALSE or integer 0