Index: src/wp-includes/plugin.php
===================================================================
--- src/wp-includes/plugin.php	(revision 38522)
+++ src/wp-includes/plugin.php	(working copy)
@@ -22,17 +22,20 @@
  */
 
 // Initialize the filter globals.
-global $wp_filter, $wp_actions, $merged_filters, $wp_current_filter;
+require( ABSPATH . WPINC . '/class-wp-hook.php' );
 
-if ( ! isset( $wp_filter ) )
+/** @var WP_Hook[] $wp_filter */
+global $wp_filter, $wp_actions, $wp_current_filter;
+
+if ( $wp_filter ) {
+	$wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter );
+} else {
 	$wp_filter = array();
+}
 
 if ( ! isset( $wp_actions ) )
 	$wp_actions = array();
 
-if ( ! isset( $merged_filters ) )
-	$merged_filters = array();
-
 if ( ! isset( $wp_current_filter ) )
 	$wp_current_filter = array();
 
@@ -89,8 +92,6 @@
  * @since 0.71
  *
  * @global array $wp_filter      A multidimensional array of all hooks and the callbacks hooked to them.
- * @global array $merged_filters Tracks the tags that need to be merged for later. If the hook is added,
- *                               it doesn't need to run through that process.
  *
  * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
  * @param callable $function_to_add The callback to be run when the filter is applied.
@@ -103,11 +104,11 @@
  * @return true
  */
 function add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ) {
-	global $wp_filter, $merged_filters;
-
-	$idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
-	$wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
-	unset( $merged_filters[ $tag ] );
+	global $wp_filter;
+	if ( ! isset( $wp_filter[ $tag ] ) ) {
+		$wp_filter[ $tag ] = new WP_Hook();
+	}
+	$wp_filter[ $tag ]->add_filter( $tag, $function_to_add, $priority, $accepted_args );
 	return true;
 }
 
@@ -128,38 +129,13 @@
  *                   return value.
  */
 function has_filter($tag, $function_to_check = false) {
-	// Don't reset the internal array pointer
-	$wp_filter = $GLOBALS['wp_filter'];
-
-	$has = ! empty( $wp_filter[ $tag ] );
-
-	// Make sure at least one priority has a filter callback
-	if ( $has ) {
-		$exists = false;
-		foreach ( $wp_filter[ $tag ] as $callbacks ) {
-			if ( ! empty( $callbacks ) ) {
-				$exists = true;
-				break;
-			}
-		}
-
-		if ( ! $exists ) {
-			$has = false;
-		}
-	}
-
-	if ( false === $function_to_check || false === $has )
-		return $has;
+	global $wp_filter;
 
-	if ( !$idx = _wp_filter_build_unique_id($tag, $function_to_check, false) )
+	if ( ! isset( $wp_filter[ $tag ] ) ) {
 		return false;
-
-	foreach ( (array) array_keys($wp_filter[$tag]) as $priority ) {
-		if ( isset($wp_filter[$tag][$priority][$idx]) )
-			return $priority;
 	}
 
-	return false;
+	return $wp_filter[ $tag ]->has_filter( $tag, $function_to_check );
 }
 
 /**
@@ -190,7 +166,6 @@
  * @since 0.71
  *
  * @global array $wp_filter         Stores all of the filters.
- * @global array $merged_filters    Merges the filter hooks using this function.
  * @global array $wp_current_filter Stores the list of current filters with the current one last.
  *
  * @param string $tag     The name of the filter hook.
@@ -199,7 +174,7 @@
  * @return mixed The filtered value after all hooked functions are applied to it.
  */
 function apply_filters( $tag, $value ) {
-	global $wp_filter, $merged_filters, $wp_current_filter;
+	global $wp_filter, $wp_current_filter;
 
 	$args = array();
 
@@ -219,29 +194,17 @@
 	if ( !isset($wp_filter['all']) )
 		$wp_current_filter[] = $tag;
 
-	// Sort.
-	if ( !isset( $merged_filters[ $tag ] ) ) {
-		ksort($wp_filter[$tag]);
-		$merged_filters[ $tag ] = true;
-	}
-
-	reset( $wp_filter[ $tag ] );
-
 	if ( empty($args) )
 		$args = func_get_args();
 
-	do {
-		foreach ( (array) current($wp_filter[$tag]) as $the_ )
-			if ( !is_null($the_['function']) ){
-				$args[1] = $value;
-				$value = call_user_func_array($the_['function'], array_slice($args, 1, (int) $the_['accepted_args']));
-			}
+	// don't pass the tag name to WP_Hook
+	array_shift( $args );
 
-	} while ( next($wp_filter[$tag]) !== false );
+	$filtered = $wp_filter[ $tag ]->apply_filters( $value, $args );
 
 	array_pop( $wp_current_filter );
 
-	return $value;
+	return $filtered;
 }
 
 /**
@@ -253,7 +216,6 @@
  * functions hooked to `$tag` are supplied using an array.
  *
  * @global array $wp_filter         Stores all of the filters
- * @global array $merged_filters    Merges the filter hooks using this function.
  * @global array $wp_current_filter Stores the list of current filters with the current one last
  *
  * @param string $tag  The name of the filter hook.
@@ -261,7 +223,7 @@
  * @return mixed The filtered value after all hooked functions are applied to it.
  */
 function apply_filters_ref_array($tag, $args) {
-	global $wp_filter, $merged_filters, $wp_current_filter;
+	global $wp_filter, $wp_current_filter;
 
 	// Do 'all' actions first
 	if ( isset($wp_filter['all']) ) {
@@ -279,24 +241,11 @@
 	if ( !isset($wp_filter['all']) )
 		$wp_current_filter[] = $tag;
 
-	// Sort
-	if ( !isset( $merged_filters[ $tag ] ) ) {
-		ksort($wp_filter[$tag]);
-		$merged_filters[ $tag ] = true;
-	}
-
-	reset( $wp_filter[ $tag ] );
-
-	do {
-		foreach ( (array) current($wp_filter[$tag]) as $the_ )
-			if ( !is_null($the_['function']) )
-				$args[0] = call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));
-
-	} while ( next($wp_filter[$tag]) !== false );
+	$filtered = $wp_filter[ $tag ]->apply_filters( $args[0], $args );
 
 	array_pop( $wp_current_filter );
 
-	return $args[0];
+	return $filtered;
 }
 
 /**
@@ -313,7 +262,6 @@
  * @since 1.2.0
  *
  * @global array $wp_filter         Stores all of the filters
- * @global array $merged_filters    Merges the filter hooks using this function.
  *
  * @param string   $tag                The filter hook to which the function to be removed is hooked.
  * @param callable $function_to_remove The name of the function which should be removed.
@@ -321,19 +269,14 @@
  * @return bool    Whether the function existed before it was removed.
  */
 function remove_filter( $tag, $function_to_remove, $priority = 10 ) {
-	$function_to_remove = _wp_filter_build_unique_id( $tag, $function_to_remove, $priority );
-
-	$r = isset( $GLOBALS['wp_filter'][ $tag ][ $priority ][ $function_to_remove ] );
+	global $wp_filter;
 
-	if ( true === $r ) {
-		unset( $GLOBALS['wp_filter'][ $tag ][ $priority ][ $function_to_remove ] );
-		if ( empty( $GLOBALS['wp_filter'][ $tag ][ $priority ] ) ) {
-			unset( $GLOBALS['wp_filter'][ $tag ][ $priority ] );
+	$r = false;
+	if ( isset( $wp_filter[ $tag ] ) ) {
+		$r = $wp_filter[ $tag ]->remove_filter( $tag, $function_to_remove, $priority );
+		if ( ! $wp_filter[ $tag ]->callbacks ) {
+			unset( $wp_filter[ $tag ] );
 		}
-		if ( empty( $GLOBALS['wp_filter'][ $tag ] ) ) {
-			$GLOBALS['wp_filter'][ $tag ] = array();
-		}
-		unset( $GLOBALS['merged_filters'][ $tag ] );
 	}
 
 	return $r;
@@ -344,26 +287,22 @@
  *
  * @since 2.7.0
  *
- * @global array $wp_filter         Stores all of the filters
- * @global array $merged_filters    Merges the filter hooks using this function.
+ * @global array $wp_filter  Stores all of the filters
  *
  * @param string   $tag      The filter to remove hooks from.
  * @param int|bool $priority Optional. The priority number to remove. Default false.
  * @return true True when finished.
  */
 function remove_all_filters( $tag, $priority = false ) {
-	global $wp_filter, $merged_filters;
+	global $wp_filter;
 
 	if ( isset( $wp_filter[ $tag ]) ) {
-		if ( false === $priority ) {
-			$wp_filter[ $tag ] = array();
-		} elseif ( isset( $wp_filter[ $tag ][ $priority ] ) ) {
-			$wp_filter[ $tag ][ $priority ] = array();
+		$wp_filter[ $tag ]->remove_all_filters( $priority );
+		if ( ! $wp_filter[ $tag ]->has_filters() ) {
+			unset( $wp_filter[ $tag ] );
 		}
 	}
 
-	unset( $merged_filters[ $tag ] );
-
 	return true;
 }
 
@@ -473,7 +412,6 @@
  *
  * @global array $wp_filter         Stores all of the filters
  * @global array $wp_actions        Increments the amount of times action was triggered.
- * @global array $merged_filters    Merges the filter hooks using this function.
  * @global array $wp_current_filter Stores the list of current filters with the current one last
  *
  * @param string $tag     The name of the action to be executed.
@@ -481,7 +419,7 @@
  *                        functions hooked to the action. Default empty.
  */
 function do_action($tag, $arg = '') {
-	global $wp_filter, $wp_actions, $merged_filters, $wp_current_filter;
+	global $wp_filter, $wp_actions, $wp_current_filter;
 
 	if ( ! isset($wp_actions[$tag]) )
 		$wp_actions[$tag] = 1;
@@ -512,20 +450,7 @@
 	for ( $a = 2, $num = func_num_args(); $a < $num; $a++ )
 		$args[] = func_get_arg($a);
 
-	// Sort
-	if ( !isset( $merged_filters[ $tag ] ) ) {
-		ksort($wp_filter[$tag]);
-		$merged_filters[ $tag ] = true;
-	}
-
-	reset( $wp_filter[ $tag ] );
-
-	do {
-		foreach ( (array) current($wp_filter[$tag]) as $the_ )
-			if ( !is_null($the_['function']) )
-				call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));
-
-	} while ( next($wp_filter[$tag]) !== false );
+	$wp_filter[ $tag ]->do_action( $args );
 
 	array_pop($wp_current_filter);
 }
@@ -558,14 +483,13 @@
  *                  functions hooked to $tag< are supplied using an array.
  * @global array $wp_filter         Stores all of the filters
  * @global array $wp_actions        Increments the amount of times action was triggered.
- * @global array $merged_filters    Merges the filter hooks using this function.
  * @global array $wp_current_filter Stores the list of current filters with the current one last
  *
  * @param string $tag  The name of the action to be executed.
  * @param array  $args The arguments supplied to the functions hooked to `$tag`.
  */
 function do_action_ref_array($tag, $args) {
-	global $wp_filter, $wp_actions, $merged_filters, $wp_current_filter;
+	global $wp_filter, $wp_actions, $wp_current_filter;
 
 	if ( ! isset($wp_actions[$tag]) )
 		$wp_actions[$tag] = 1;
@@ -588,20 +512,7 @@
 	if ( !isset($wp_filter['all']) )
 		$wp_current_filter[] = $tag;
 
-	// Sort
-	if ( !isset( $merged_filters[ $tag ] ) ) {
-		ksort($wp_filter[$tag]);
-		$merged_filters[ $tag ] = true;
-	}
-
-	reset( $wp_filter[ $tag ] );
-
-	do {
-		foreach ( (array) current($wp_filter[$tag]) as $the_ )
-			if ( !is_null($the_['function']) )
-				call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));
-
-	} while ( next($wp_filter[$tag]) !== false );
+	$wp_filter[ $tag ]->do_action( $args );
 
 	array_pop($wp_current_filter);
 }
@@ -923,13 +834,7 @@
 function _wp_call_all_hook($args) {
 	global $wp_filter;
 
-	reset( $wp_filter['all'] );
-	do {
-		foreach ( (array) current($wp_filter['all']) as $the_ )
-			if ( !is_null($the_['function']) )
-				call_user_func_array($the_['function'], $args);
-
-	} while ( next($wp_filter['all']) !== false );
+	$wp_filter['all']->do_all_hook( $args );
 }
 
 /**
Index: tests/phpunit/includes/functions.php
===================================================================
--- tests/phpunit/includes/functions.php	(revision 38522)
+++ tests/phpunit/includes/functions.php	(working copy)
@@ -21,18 +21,14 @@
 
 // For adding hooks before loading WP
 function tests_add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1) {
-	global $wp_filter, $merged_filters;
+	global $wp_filter;
 
 	$idx = _test_filter_build_unique_id($tag, $function_to_add, $priority);
 	$wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
-	unset( $merged_filters[ $tag ] );
 	return true;
 }
 
 function _test_filter_build_unique_id($tag, $function, $priority) {
-	global $wp_filter;
-	static $filter_id_count = 0;
-
 	if ( is_string($function) )
 		return $function;
 
Index: tests/phpunit/includes/testcase.php
===================================================================
--- tests/phpunit/includes/testcase.php	(revision 38522)
+++ tests/phpunit/includes/testcase.php	(working copy)
@@ -223,10 +223,14 @@
 	 * @return void
 	 */
 	protected function _backup_hooks() {
-		$globals = array( 'merged_filters', 'wp_actions', 'wp_current_filter', 'wp_filter' );
+		$globals = array( 'wp_actions', 'wp_current_filter' );
 		foreach ( $globals as $key ) {
 			self::$hooks_saved[ $key ] = $GLOBALS[ $key ];
 		}
+		self::$hooks_saved['wp_filter'] = array();
+		foreach ( $GLOBALS['wp_filter'] as $hook_name => $hook_object ) {
+			self::$hooks_saved['wp_filter'][ $hook_name ] = clone $hook_object;
+		}
 	}
 
 	/**
@@ -240,12 +244,18 @@
 	 * @return void
 	 */
 	protected function _restore_hooks() {
-		$globals = array( 'merged_filters', 'wp_actions', 'wp_current_filter', 'wp_filter' );
+		$globals = array( 'wp_actions', 'wp_current_filter' );
 		foreach ( $globals as $key ) {
 			if ( isset( self::$hooks_saved[ $key ] ) ) {
 				$GLOBALS[ $key ] = self::$hooks_saved[ $key ];
 			}
 		}
+		if ( isset( self::$hooks_saved['wp_filter'] ) ) {
+			$GLOBALS['wp_filter'] = array();
+			foreach ( self::$hooks_saved['wp_filter'] as $hook_name => $hook_object ) {
+				$GLOBALS['wp_filter'][ $hook_name ] = clone $hook_object;
+			}
+		}
 	}
 
 	static function flush_cache() {
Index: tests/phpunit/tests/actions.php
===================================================================
--- tests/phpunit/tests/actions.php	(revision 38522)
+++ tests/phpunit/tests/actions.php	(working copy)
@@ -114,6 +114,45 @@
 		$this->assertEquals( array( $val1 ), array_pop( $argsvar2 ) );
 	}
 
+	/**
+	 * Test that multiple callbacks receive the correct number of args even when the number
+	 * is less than, or greater than previous hooks.
+	 *
+	 * @see https://core.trac.wordpress.org/ticket/17817#comment:72
+	 * @ticket 17817
+	 */
+	function test_action_args_3() {
+		$a1 = new MockAction();
+		$a2 = new MockAction();
+		$a3 = new MockAction();
+		$tag = rand_str();
+		$val1 = rand_str();
+		$val2 = rand_str();
+
+		// a1 accepts two arguments, a2 doesn't, a3 accepts two arguments
+		add_action( $tag, array( &$a1, 'action' ), 10, 2 );
+		add_action( $tag, array( &$a2, 'action' ) );
+		add_action( $tag, array( &$a3, 'action' ), 10, 2 );
+		// call the action with two arguments
+		do_action( $tag, $val1, $val2 );
+
+		$call_count = $a1->get_call_count();
+		// a1 should be called with both args
+		$this->assertEquals( 1, $call_count );
+		$argsvar1 = $a1->get_args();
+		$this->assertEquals( array( $val1, $val2 ), array_pop( $argsvar1 ) );
+
+		// a2 should be called with one only
+		$this->assertEquals( 1, $a2->get_call_count() );
+		$argsvar2 = $a2->get_args();
+		$this->assertEquals( array( $val1 ), array_pop( $argsvar2 ) );
+
+		// a3 should be called with both args
+		$this->assertEquals( 1, $a3->get_call_count() );
+		$argsvar3 = $a3->get_args();
+		$this->assertEquals( array( $val1, $val2 ), array_pop( $argsvar3 ) );
+	}
+
 	function test_action_priority() {
 		$a = new MockAction();
 		$tag = rand_str();
@@ -258,6 +297,120 @@
 	}
 
 	/**
+	 * @ticket 17817
+	 */
+	function test_action_recursion() {
+		$tag = rand_str();
+		$a = new MockAction();
+		$b = new MockAction();
+
+		add_action( $tag, array( $a, 'action' ), 11, 1 );
+		add_action( $tag, array( $b, 'action' ), 13, 1 );
+		add_action( $tag, array( $this, 'action_that_causes_recursion' ), 12, 1 );
+		do_action( $tag, $tag );
+
+		$this->assertEquals( 2, $a->get_call_count(), 'recursive actions should call all callbacks with earlier priority' );
+		$this->assertEquals( 2, $b->get_call_count(), 'recursive actions should call callbacks with later priority' );
+	}
+
+	function action_that_causes_recursion( $tag ) {
+		static $recursing = false;
+		if ( ! $recursing ) {
+			$recursing = true;
+			do_action( $tag, $tag );
+		}
+		$recursing = false;
+	}
+
+	/**
+	 * @ticket 9968
+	 * @ticket 17817
+	 */
+	function test_action_callback_manipulation_while_running() {
+		$tag = rand_str();
+		$a = new MockAction();
+		$b = new MockAction();
+		$c = new MockAction();
+		$d = new MockAction();
+		$e = new MockAction();
+
+		add_action( $tag, array( $a, 'action' ), 11, 2 );
+		add_action( $tag, array( $this, 'action_that_manipulates_a_running_hook' ), 12, 2 );
+		add_action( $tag, array( $b, 'action' ), 12, 2 );
+
+		do_action( $tag, $tag, array( $a, $b, $c, $d, $e ) );
+		do_action( $tag, $tag, array( $a, $b, $c, $d, $e ) );
+
+		$this->assertEquals( 2, $a->get_call_count(), 'callbacks should run unless otherwise instructed' );
+		$this->assertEquals( 1, $b->get_call_count(), 'callback removed by same priority callback should still get called' );
+		$this->assertEquals( 1, $c->get_call_count(), 'callback added by same priority callback should not get called' );
+		$this->assertEquals( 2, $d->get_call_count(), 'callback added by earlier priority callback should get called' );
+		$this->assertEquals( 1, $e->get_call_count(), 'callback added by later priority callback should not get called' );
+	}
+
+	function action_that_manipulates_a_running_hook( $tag, $mocks ) {
+		remove_action( $tag, array( $mocks[ 1 ], 'action' ), 12, 2 );
+		add_action( $tag, array( $mocks[ 2 ], 'action' ), 12, 2 );
+		add_action( $tag, array( $mocks[ 3 ], 'action' ), 13, 2 );
+		add_action( $tag, array( $mocks[ 4 ], 'action' ), 10, 2 );
+	}
+
+	/**
+	 * @ticket 17817
+	 *
+	 * This specificaly addresses the concern raised at
+	 * https://core.trac.wordpress.org/ticket/17817#comment:52
+	 */
+	function test_remove_anonymous_callback() {
+		$tag = rand_str();
+		$a = new MockAction();
+		add_action( $tag, array( $a, 'action' ), 12, 1 );
+		$this->assertTrue( has_action( $tag ) );
+
+		$hook = $GLOBALS['wp_filter'][ $tag ];
+
+		// From http://wordpress.stackexchange.com/a/57088/6445
+		foreach ( $hook as $priority => $filter ) {
+			foreach ( $filter as $identifier => $function ) {
+				if ( is_array( $function )
+					&& is_a( $function['function'][ 0 ], 'MockAction' )
+					&& 'action' === $function['function'][ 1 ]
+				) {
+					remove_filter(
+						$tag,
+						array( $function['function'][ 0 ], 'action' ),
+						$priority
+					);
+				}
+			}
+		}
+
+		$this->assertFalse( has_action( $tag ) );
+	}
+
+
+	/**
+	 * Test the ArrayAccess methods of WP_Hook
+	 *
+	 * @ticket 17817
+	 */
+	function test_array_access_of_wp_filter_global() {
+		global $wp_filter;
+		$tag = rand_str();
+
+		add_action( $tag, '__return_null', 11, 1 );
+
+		$this->assertTrue( isset( $wp_filter[ $tag ][ 11 ] ) );
+		$this->assertArrayHasKey( '__return_null', $wp_filter[ $tag ][ 11 ] );
+
+		unset( $wp_filter[ $tag ][ 11 ] );
+		$this->assertFalse( has_action( $tag, '__return_null' ) );
+
+		$wp_filter[ $tag ][ 11 ] = array( '__return_null' => array( 'function' => '__return_null', 'accepted_args' => 1 ) );
+		$this->assertEquals( 11, has_action( $tag, '__return_null' ) );
+	}
+
+	/**
 	 * Make sure current_action() behaves as current_filter()
 	 *
 	 * @ticket 14994
Index: tests/phpunit/tests/filters.php
===================================================================
--- tests/phpunit/tests/filters.php	(revision 38522)
+++ tests/phpunit/tests/filters.php	(working copy)
@@ -296,27 +296,6 @@
 	}
 
 	/**
-	 * @ticket 29070
-	 */
-	 function test_has_filter_doesnt_reset_wp_filter() {
-	 	add_action( 'action_test_has_filter_doesnt_reset_wp_filter', '__return_null', 1 );
-	 	add_action( 'action_test_has_filter_doesnt_reset_wp_filter', '__return_null', 2 );
-	 	add_action( 'action_test_has_filter_doesnt_reset_wp_filter', '__return_null', 3 );
-	 	add_action( 'action_test_has_filter_doesnt_reset_wp_filter', array( $this, '_action_test_has_filter_doesnt_reset_wp_filter' ), 4 );
-
-	 	do_action( 'action_test_has_filter_doesnt_reset_wp_filter' );
-	 }
-	 function _action_test_has_filter_doesnt_reset_wp_filter() {
-	 	global $wp_filter;
-
-	 	has_action( 'action_test_has_filter_doesnt_reset_wp_filter', '_function_that_doesnt_exist' );
-
-		$filters = current( $wp_filter['action_test_has_filter_doesnt_reset_wp_filter'] );
-	 	$the_ = current( $filters );
-	 	$this->assertEquals( $the_['function'], array( $this, '_action_test_has_filter_doesnt_reset_wp_filter' ) );
-	 }
-
-	/**
 	 * @ticket 10441
 	 * @expectedDeprecated tests_apply_filters_deprecated
 	 */
Index: tests/phpunit/tests/post/types.php
===================================================================
--- tests/phpunit/tests/post/types.php	(revision 38522)
+++ tests/phpunit/tests/post/types.php	(working copy)
@@ -446,7 +446,7 @@
 
 		$this->assertSame( 1, count( $wp_filter['future_foo'] ) );
 		$this->assertTrue( unregister_post_type( 'foo' ) );
-		$this->assertSame( array(), $wp_filter['future_foo'] );
+		$this->assertArrayNotHasKey( 'future_foo', $wp_filter );
 	}
 
 	/**
@@ -462,7 +462,7 @@
 
 		$this->assertSame( 1, count( $wp_filter['add_meta_boxes_foo'] ) );
 		$this->assertTrue( unregister_post_type( 'foo' ) );
-		$this->assertSame( array(), $wp_filter['add_meta_boxes_foo'] );
+		$this->assertArrayNotHasKey( 'add_meta_boxes_foo', $wp_filter );
 	}
 
 	/**
Index: tests/phpunit/tests/taxonomy.php
===================================================================
--- tests/phpunit/tests/taxonomy.php	(revision 38522)
+++ tests/phpunit/tests/taxonomy.php	(working copy)
@@ -697,7 +697,7 @@
 
 		$this->assertSame( 1, count( $wp_filter['wp_ajax_add-foo'] ) );
 		$this->assertTrue( unregister_taxonomy( 'foo' ) );
-		$this->assertSame( array(), $wp_filter['wp_ajax_add-foo'] );
+		$this->assertArrayNotHasKey( 'wp_ajax_add-foo', $wp_filter );
 	}
 
 	/**