diff -u b/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
--- b/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -310,6 +310,7 @@
   $items['taxonomy/autocomplete/%'] = array(
     'title' => 'Autocomplete taxonomy',
     'page callback' => 'taxonomy_autocomplete',
+    'page arguments' => array(2),
     'access arguments' => array('access content'),
     'type' => MENU_CALLBACK,
     'file' => 'taxonomy.pages.inc',
diff -u b/core/modules/views/includes/ajax.inc b/core/modules/views/includes/ajax.inc
--- b/core/modules/views/includes/ajax.inc
+++ b/core/modules/views/includes/ajax.inc
@@ -281,11 +281,19 @@
 /**
  * Page callback for views user autocomplete.
  *
+ * @param string $string
+ *   (optional) A comma-separated list of user names entered in the
+ *   autocomplete form element. If not passed, it is taken from the 'q' query
+ *   string parameter.
+ *
  * @return Symfony\Component\HttpFoundation\JsonResponse
  */
-function views_ajax_autocomplete_user() {
+function views_ajax_autocomplete_user($string = NULL) {
+  if (!isset($string)) {
+    $string = drupal_container()->get('request')->query->get('q');
+  }
   // The user enters a comma-separated list of user name. We only autocomplete the last name.
-  $array = drupal_explode_tags(drupal_container()->get('request')->query->get('q'));
+  $array = drupal_explode_tags($string);
 
   // Fetch last name
   $last_string = trim(array_pop($array));
diff -u b/core/modules/views/views_ui/admin.inc b/core/modules/views/views_ui/admin.inc
--- b/core/modules/views/views_ui/admin.inc
+++ b/core/modules/views/views_ui/admin.inc
@@ -2095,9 +2095,11 @@
 /**
  * Page callback for views tag autocomplete
  */
-function views_ui_autocomplete_tag() {
+function views_ui_autocomplete_tag($string = NULL) {
   $matches = array();
-  $string = drupal_container()->get('request')->query->get('q');
+  if (!isset($string)) {
+    $string = drupal_container()->get('request')->query->get('q');
+  }
   // get matches from default views:
   $views = views_get_all_views();
   foreach ($views as $view) {
only in patch2:
unchanged:
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php
@@ -39,7 +39,7 @@ function setUp() {
    */
   function testUserAutocomplete() {
     $this->drupalLogin($this->account);
-    $this->drupalGet('user/autocomplete/' . $this->account->name);
+    $this->drupalGet('user/autocomplete', array('query' => array('q' => $this->account->name)));
     $this->assertRaw($this->account->name);
     $this->assertNoText('registry initialized', 'The registry was not initialized');
   }
only in patch2:
unchanged:
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/TermTest.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/TermTest.php
@@ -205,13 +205,13 @@ function testNodeTermCreationAndDeletion() {
     // Test autocomplete on term 3, which contains a comma.
     // The term will be quoted, and the " will be encoded in unicode (\u0022).
     $input = substr($term_objects['term3']->name, 0, 3);
-    $json = $this->drupalGet('taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name . '/' . $input);
+    $json = $this->drupalGet('taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name, array('query' => array('q' => $input)));
     $this->assertEqual($json, '{"\u0022' . $term_objects['term3']->name . '\u0022":"' . $term_objects['term3']->name . '"}', format_string('Autocomplete returns term %term_name after typing the first 3 letters.', array('%term_name' => $term_objects['term3']->name)));
 
     // Test autocomplete on term 4 - it is alphanumeric only, so no extra
     // quoting.
     $input = substr($term_objects['term4']->name, 0, 3);
-    $this->drupalGet('taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name . '/' . $input);
+    $this->drupalGet('taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name, array('query' => array('q' => $input)));
     $this->assertRaw('{"' . $term_objects['term4']->name . '":"' . $term_objects['term4']->name . '"}', format_string('Autocomplete returns term %term_name after typing the first 3 letters.', array('%term_name' => $term_objects['term4']->name)));
 
     // Test taxonomy autocomplete with a nonexistent field.
@@ -219,7 +219,7 @@ function testNodeTermCreationAndDeletion() {
     $tag = $this->randomName();
     $message = t("Taxonomy field @field_name not found.", array('@field_name' => $field_name));
     $this->assertFalse(field_info_field($field_name), format_string('Field %field_name does not exist.', array('%field_name' => $field_name)));
-    $this->drupalGet('taxonomy/autocomplete/' . $field_name . '/' . $tag);
+    $this->drupalGet('taxonomy/autocomplete/' . $field_name, array('query' => array('q' => $tag)));
     $this->assertRaw($message, 'Autocomplete returns correct error message when the taxonomy field does not exist.');
   }
 
@@ -243,10 +243,9 @@ function testTermAutocompletion() {
     // Try to autocomplete a term name that matches both terms.
     // We should get both term in a json encoded string.
     $input = '10/';
-    $path = 'taxonomy/autocomplete/taxonomy_';
-    $path .= $this->vocabulary->machine_name . '/' . $input;
+    $path = 'taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name;
     // The result order is not guaranteed, so check each term separately.
-    $result = $this->drupalGet($path);
+    $result = $this->drupalGet($path, array('query' => array('q' => $input)));
     $data = drupal_json_decode($result);
     $this->assertEqual($data[$first_term->name], check_plain($first_term->name), 'Autocomplete returned the first matching term');
     $this->assertEqual($data[$second_term->name], check_plain($second_term->name), 'Autocomplete returned the second matching term');
@@ -254,17 +253,15 @@ function testTermAutocompletion() {
     // Try to autocomplete a term name that matches first term.
     // We should only get the first term in a json encoded string.
     $input = '10/16';
-    $url = 'taxonomy/autocomplete/taxonomy_';
-    $url .= $this->vocabulary->machine_name . '/' . $input;
-    $this->drupalGet($url);
+    $path = 'taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name;
+    $this->drupalGet($path, array('query' => array('q' => $input)));
     $target = array($first_term->name => check_plain($first_term->name));
     $this->assertRaw(drupal_json_encode($target), 'Autocomplete returns only the expected matching term.');
 
     // Try to autocomplete a term name with both a comma and a slash.
     $input = '"term with, comma and / a';
-    $url = 'taxonomy/autocomplete/taxonomy_';
-    $url .= $this->vocabulary->machine_name . '/' . $input;
-    $this->drupalGet($url);
+    $path = 'taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name;
+    $this->drupalGet($path, array('query' => array('q' => $input)));
     $n = $third_term->name;
     // Term names containing commas or quotes must be wrapped in quotes.
     if (strpos($third_term->name, ',') !== FALSE || strpos($third_term->name, '"') !== FALSE) {
only in patch2:
unchanged:
--- a/core/modules/user/lib/Drupal/user/Tests/UserAutocompleteTest.php
+++ b/core/modules/user/lib/Drupal/user/Tests/UserAutocompleteTest.php
@@ -35,13 +35,13 @@ function setUp() {
   function testUserAutocomplete() {
     // Check access from unprivileged user, should be denied.
     $this->drupalLogin($this->unprivileged_user);
-    $this->drupalGet('user/autocomplete/' . $this->unprivileged_user->name[0]);
+    $this->drupalGet('user/autocomplete', array('query' => array('q' => $this->unprivileged_user->name[0])));
     $this->assertResponse(403, 'Autocompletion access denied to user without permission.');
 
     // Check access from privileged user.
     $this->drupalLogout();
     $this->drupalLogin($this->privileged_user);
-    $this->drupalGet('user/autocomplete/' . $this->unprivileged_user->name[0]);
+    $this->drupalGet('user/autocomplete', array('query' => array('q' => $this->unprivileged_user->name[0])));
     $this->assertResponse(200, 'Autocompletion access allowed.');
 
     // Using first letter of the user's name, make sure the user's full name is in the results.