Index: src/wp-includes/meta.php
===================================================================
--- src/wp-includes/meta.php	(revision 37961)
+++ src/wp-includes/meta.php	(working copy)
@@ -1075,23 +1075,37 @@
 		}
 	}
 
+	$object_subtype = '';
+
+	if ( ! empty( $args['object_subtype'] ) ) {
+		$object_subtype = $args['object_subtype'];
+	}
+
 	// Back-compat: old sanitize and auth callbacks applied to all of an object type
 	if ( $has_old_sanitize_cb ) {
-		add_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
-		add_filter( "auth_{$object_type}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
+				add_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
+				add_filter( "auth_{$object_type}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
 	} else {
+		// Back-compat: old sanitize and auth callbacks applied to all of an object type
 		if ( is_callable( $args['sanitize_callback'] ) ) {
-			add_filter( "sanitize_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
+			if ( empty( $object_subtype ) || $has_old_sanitize_cb ) {
+				add_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
+			} else {
+				add_filter( "sanitize_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
+			}
 		}
 
 		if ( is_callable( $args['auth_callback'] ) ) {
-			add_filter( "auth_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
+			if ( empty( $object_subtype ) || $has_old_sanitize_cb ) {
+				add_filter( "auth_{$object_type}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
+			} else {
+				add_filter( "auth_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
+			}
 		}
 	}
 
 	// Global registry only contains meta keys registered in the new way with a subtype.
-	if ( ! empty( $args['object_subtype'] ) ) {
-		$object_subtype = $args['object_subtype'];
+	if ( ! empty( $object_subtype ) ) {
 		$wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ] = $args;
 
 		return true;
@@ -1234,7 +1248,7 @@
 		return $data;
 	}
 
-	$data = get_metadata( $object_type, $object_id, $meta_key );
+	$data = get_metadata( $object_type, $object_id );
 
 	$meta_keys = get_registered_meta_keys( $object_type, $object_subtype );
 	$registered_data = array();
Index: tests/phpunit/tests/meta/registerMeta.php
===================================================================
--- tests/phpunit/tests/meta/registerMeta.php	(nonexistent)
+++ tests/phpunit/tests/meta/registerMeta.php	(working copy)
@@ -0,0 +1,354 @@
+<?php
+/**
+ * @group meta
+ */
+class Tests_Meta_Register_Meta extends WP_UnitTestCase {
+	protected static $editor_id;
+	protected static $post_id;
+	protected static $comment_id;
+
+	public static function wpSetUpBeforeClass( $factory ) {
+		self::$editor_id = $factory->user->create( array( 'role' => 'editor' ) );
+		self::$post_id = $factory->post->create();
+		self::$comment_id = $factory->comment->create( array( 'comment_post_ID' => self::$post_id ) );
+	}
+
+	public static function wpTearDownAfterClass() {
+		self::delete_user( self::$editor_id );
+		wp_delete_comment( self::$comment_id, true );
+		wp_delete_post( self::$post_id, true );
+	}
+
+	function setUp() {
+		parent::setUp();
+		wp_set_current_user( self::$editor_id );
+
+	}
+
+	public function _old_sanitize_meta_cb( $meta_value, $meta_key, $meta_type ) {
+		return $meta_key . ' old sanitized';
+	}
+
+	public function _new_sanitize_meta_cb( $meta_value, $meta_key, $object_type, $object_subtype ) {
+		return $meta_key . ' new sanitized';
+	}
+
+	public function test_register_meta_with_valid_object_type_and_object_subtype_returns_true() {
+		$result = register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+		unregister_meta_key( 'post', 'post', 'flight_number' );
+
+		$this->assertTrue( $result );
+	}
+
+	// @todo remove object_subtype from the meta key's array?
+	public function test_register_meta_with_post_object_type_and_subtype_populates_wp_meta_keys() {
+		global $wp_meta_keys;
+
+		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+		$actual = $wp_meta_keys;
+		unregister_meta_key( 'post', 'post', 'flight_number' );
+
+		$expected = array(
+			'post' => array(
+				'post' => array(
+					'flight_number' => array(
+						'object_subtype' => 'post',
+						'type' => 'string',
+						'description' => '',
+						'single' => false,
+						'sanitize_callback' => null,
+						'auth_callback' => '__return_true',
+						'show_in_rest' => false,
+					),
+				),
+			),
+		);
+
+		$this->assertEquals( $actual, $expected );
+	}
+
+	public function test_register_meta_with_post_object_type_and_unregistered_subtype_populates_wp_meta_keys() {
+		global $wp_meta_keys;
+
+		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'not_a_post_type' ) );
+		$actual = $wp_meta_keys;
+		unregister_meta_key( 'post', 'not_a_post_type', 'flight_number' );
+
+		$expected = array(
+			'post' => array(
+				'not_a_post_type' => array(
+					'flight_number' => array(
+						'object_subtype' => 'not_a_post_type',
+						'type' => 'string',
+						'description' => '',
+						'single' => false,
+						'sanitize_callback' => null,
+						'auth_callback' => '__return_true',
+						'show_in_rest' => false,
+					),
+				),
+			),
+		);
+
+		$this->assertEquals( $actual, $expected );
+	}
+
+	// @todo remove object_subtype from the meta key's array?
+	public function test_register_meta_with_term_object_type_and_category_subtype_populates_wp_meta_keys() {
+		global $wp_meta_keys;
+		register_meta( 'term', 'category_icon', array( 'object_subtype' => 'category' ) );
+		$actual = $wp_meta_keys;
+		unregister_meta_key( 'term', 'category', 'category_icon' );
+
+		$expected = array(
+			'term' => array(
+				'category' => array(
+					'category_icon' => array(
+						'object_subtype' => 'category',
+						'type' => 'string',
+						'description' => '',
+						'single' => false,
+						'sanitize_callback' => null,
+						'auth_callback' => '__return_true',
+						'show_in_rest' => false,
+					),
+				),
+			),
+		);
+
+		$this->assertEquals( $actual, $expected );
+	}
+
+	public function test_register_meta_with_comment_object_type_and_subtype_populates_wp_meta_keys() {
+		global $wp_meta_keys;
+		register_meta( 'comment', 'comment_rating', array( 'object_subtype' => 'comment' ) );
+		$actual = $wp_meta_keys;
+		unregister_meta_key( 'comment', 'comment', 'comment_rating' );
+
+		$expected = array(
+			'comment' => array(
+				'comment' => array(
+					'comment_rating' => array(
+						'object_subtype' => 'comment',
+						'type' => 'string',
+						'description' => '',
+						'single' => false,
+						'sanitize_callback' => null,
+						'auth_callback' => '__return_true',
+						'show_in_rest' => false,
+					),
+				),
+			),
+		);
+
+		$this->assertEquals( $actual, $expected );
+	}
+
+	public function test_register_meta_with_deprecated_sanitize_callback_does_not_populate_wp_meta_keys() {
+		global $wp_meta_keys;
+
+		register_meta( 'post', 'flight_number', array( $this, '_old_sanitize_meta_cb' ) );
+		$actual = $wp_meta_keys;
+		remove_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb') );
+		remove_filter( 'auth_post_meta_flight_number', '__return_true');
+
+		$this->assertEquals( array(), $actual );
+	}
+
+	public function test_register_meta_with_deprecated_sanitize_callback_param_returns_false() {
+		$actual = register_meta( 'post', 'flight_number', array( $this, '_old_sanitize_meta_cb' ) );
+
+		remove_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb') );
+		remove_filter( 'auth_post_meta_flight_number', '__return_true');
+
+		$this->assertFalse( $actual );
+	}
+
+	public function test_register_meta_with_deprecated_sanitize_callback_parameter_passes_through_filter() {
+		register_meta( 'post', 'old_sanitized_key', array( $this, '_old_sanitize_meta_cb' ) );
+		$meta = sanitize_meta( 'old_sanitized_key', 'unsanitized', 'post', 'post' );
+
+		remove_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb') );
+		remove_filter( 'auth_post_meta_flight_number', '__return_true');
+
+		$this->assertEquals( 'old_sanitized_key old sanitized', $meta );
+	}
+
+	public function test_register_meta_with_current_sanitize_callback_populates_wp_meta_keys() {
+		global $wp_meta_keys;
+		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post', 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		$actual = $wp_meta_keys;
+		unregister_meta_key( 'post', 'post', 'flight_number' );
+
+		$expected = array(
+			'post' => array(
+				'post' => array(
+					'flight_number' => array(
+						'object_subtype' => 'post',
+						'type' => 'string',
+						'description' => '',
+						'single' => false,
+						'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ),
+						'auth_callback' => '__return_true',
+						'show_in_rest' => false,
+					),
+				),
+			),
+		);
+		$this->assertEquals( $actual, $expected );
+	}
+
+	public function test_register_meta_with_current_sanitize_callback_returns_true() {
+		$result = register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post', 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		unregister_meta_key( 'post', 'post', 'flight_number' );
+
+		// @todo unregister_meta_key should do this for registered meta keys
+		remove_filter( 'sanitize_post_post_meta_flight_number', array( $this, '_new_sanitize_meta_cb') );
+		remove_filter( 'auth_post_post_meta_flight_number', '__return_true');
+
+		$this->assertTrue( $result );
+	}
+
+	public function test_register_meta_with_new_sanitize_callback_parameter() {
+		register_meta( 'post', 'new_sanitized_key', array( 'object_subtype' => 'post', 'sanitize_callback' => array( $this, '_new_sanitize_meta_cb' ) ) );
+		$meta = sanitize_meta( 'new_sanitized_key', 'unsanitized', 'post', 'post' );
+
+		unregister_meta_key( 'post', 'post', 'new_sanitized_key' );
+
+		// @todo unregister_meta_key should do this for registered meta keys
+		remove_filter( 'sanitize_post_post_meta_new_sanitized_key', array( $this, '_new_sanitize_meta_cb') );
+		remove_filter( 'auth_post_post_meta_new_sanitized_key', '__return_true');
+
+		$this->assertEquals( 'new_sanitized_key new sanitized', $meta );
+	}
+
+	public function test_unregister_meta_key_clears_key_from_wp_meta_keys() {
+		global $wp_meta_keys;
+		register_meta( 'post', 'registered_key', array( 'object_subtype' => 'post' ) );
+		unregister_meta_key( 'post', 'post', 'registered_key' );
+
+		$this->assertEquals( array(), $wp_meta_keys );
+	}
+
+	public function test_unregister_meta_key_with_invalid_key_returns_wp_error() {
+		$this->assertWPError( unregister_meta_key( 'post', 'post', 'not_a_registered_key' ) );
+	}
+
+	public function test_get_registered_meta_keys() {
+		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post', 'registered_key2', array( 'object_subtype' => 'post' ) );
+
+		$meta_keys = get_registered_meta_keys( 'post', 'post' );
+
+		unregister_meta_key( 'post', 'post', 'registered_key1' );
+		unregister_meta_key( 'post', 'post', 'registered_key2' );
+
+		$this->assertArrayHasKey( 'registered_key1', $meta_keys );
+		$this->assertArrayHasKey( 'registered_key2', $meta_keys );
+	}
+
+	public function test_get_registered_meta_keys_with_subtype_without_registered_keys_is_empty() {
+		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post', 'registered_key2', array( 'object_subtype' => 'post' ) );
+
+		$meta_keys = get_registered_meta_keys( 'post', 'page' );
+
+		unregister_meta_key( 'post', 'post', 'registered_key1' );
+		unregister_meta_key( 'post', 'post', 'registered_key2' );
+
+		$this->assertEmpty( $meta_keys );
+	}
+
+	public function test_get_registered_meta_keys_with_invalid_type_is_empty() {
+		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post', 'registered_key2', array( 'object_subtype' => 'post' ) );
+
+		$meta_keys = get_registered_meta_keys( 'invalid-type' );
+
+		unregister_meta_key( 'post', 'post', 'registered_key1' );
+		unregister_meta_key( 'post', 'post', 'registered_key2' );
+
+		$this->assertEmpty( $meta_keys );
+	}
+
+	public function test_get_registered_meta_keys_has_count() {
+		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post' ) );
+		register_meta( 'post', 'registered_key2', array( 'object_subtype' => 'page' ) );
+
+		$meta_keys = get_registered_meta_keys( 'post' );
+
+		$this->assertCount( 2, $meta_keys );
+	}
+
+	public function test_get_registered_meta_keys_description_arg() {
+		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post', 'description' => 'I\'m just a field, take a good look at me' ) );
+
+		$meta_keys = get_registered_meta_keys( 'post', 'post' );
+
+		$this->assertEquals( 'I\'m just a field, take a good look at me', $meta_keys['registered_key1']['description'] );
+	}
+
+	public function test_get_registered_meta_keys_invalid_arg() {
+		register_meta( 'post', 'registered_key1', array( 'object_subtype' => 'post', 'invalid_arg' => 'invalid' ) );
+
+		$meta_keys = get_registered_meta_keys( 'post', 'post' );
+
+		$this->assertArrayNotHasKey( 'invalid_arg', $meta_keys['registered_key1'] );
+	}
+
+	public function test_get_registered_metadata() {
+		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+
+		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
+
+		$meta = get_registered_metadata( 'post', 'post', self::$post_id );
+
+		$this->assertEquals( 'Oceanic 815', $meta['flight_number'][0] );
+	}
+
+	public function test_get_registered_metadata_by_key() {
+		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+
+		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
+
+		$meta = get_registered_metadata( 'post', 'post', self::$post_id, 'flight_number' );
+
+		$this->assertEquals( 'Oceanic 815', $meta[0] );
+	}
+
+	public function test_get_registered_metadata_by_key_single() {
+		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post', 'single' => true ) );
+
+		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
+
+		$meta = get_registered_metadata( 'post', 'post', self::$post_id, 'flight_number' );
+
+		$this->assertEquals( 'Oceanic 815', $meta );
+	}
+
+	public function test_get_registered_metadata_by_invalid_key() {
+		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+
+		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
+
+		$meta = get_registered_metadata( 'post', 'post', self::$post_id, 'flight_pilot' );
+
+		$this->assertWPError( $meta );
+	}
+
+	public function test_get_registered_metadata_invalid_object_type() {
+		register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
+
+		add_post_meta( self::$post_id, 'flight_number', 'Oceanic 815' );
+
+		$meta = get_registered_metadata( 'invalid-type', 'invalid-subtype', self::$post_id );
+
+		$this->assertWPError( $meta );
+	}
+
+	public function test_get_registered_metadata_invalid() {
+		$meta = get_registered_metadata( 'invalid-type', 'invalid-subtype', self::$post_id );
+
+		$this->assertWPError( $meta );
+	}
+}
Index: tests/phpunit/tests/user/capabilities.php
===================================================================
--- tests/phpunit/tests/user/capabilities.php	(revision 37961)
+++ tests/phpunit/tests/user/capabilities.php	(working copy)
@@ -772,23 +772,59 @@
 			$this->assertTrue( $admin->has_cap('add_post_meta',  $post) );
 			$this->assertTrue( $admin->has_cap('delete_post_meta',  $post) );
 
+			// Test protected key access is false
 			$this->assertFalse( $admin->has_cap('edit_post_meta', $post, '_protected') );
 			$this->assertFalse( $admin->has_cap('add_post_meta', $post, '_protected') );
 			$this->assertFalse( $admin->has_cap('delete_post_meta', $post, '_protected') );
 
-			register_meta( 'post', '_protected', array( $this, '_meta_filter' ), array( $this, '_meta_yes_you_can' ) );
+			// Register protected key and allow access
+			register_meta( 'post', '_protected', array( 'sanitize_callback' => array( $this, '_meta_filter' ), 'auth_callback' => array( $this, '_meta_yes_you_can' ) ) );
+
+			// Test protected key access is now true
 			$this->assertTrue( $admin->has_cap('edit_post_meta',  $post, '_protected') );
 			$this->assertTrue( $admin->has_cap('add_post_meta',  $post, '_protected') );
 			$this->assertTrue( $admin->has_cap('delete_post_meta',  $post, '_protected') );
 
+			// Test non protected key access is true
 			$this->assertTrue( $admin->has_cap('edit_post_meta', $post, 'not_protected') );
 			$this->assertTrue( $admin->has_cap('add_post_meta', $post, 'not_protected') );
 			$this->assertTrue( $admin->has_cap('delete_post_meta', $post, 'not_protected') );
 
-			register_meta( 'post', 'not_protected', array( $this, '_meta_filter' ), array( $this, '_meta_no_you_cant' ) );
+			// Register non protected key and disallow access
+			register_meta( 'post', 'not_protected', array( 'sanitize_callback' => array( $this, '_meta_filter' ), 'auth_callback' => array( $this, '_meta_no_you_cant' ) ) );
+
+			// Test non protected key access is now false
 			$this->assertFalse( $admin->has_cap('edit_post_meta',  $post, 'not_protected') );
 			$this->assertFalse( $admin->has_cap('add_post_meta',  $post, 'not_protected') );
 			$this->assertFalse( $admin->has_cap('delete_post_meta',  $post, 'not_protected') );
+
+			// Backwards compatibility tests for register_meta()
+
+			// Test protected key access is false
+			$this->assertFalse( $admin->has_cap('edit_post_meta', $post, '_protected_backcompat') );
+			$this->assertFalse( $admin->has_cap('add_post_meta', $post, '_protected_backcompat') );
+			$this->assertFalse( $admin->has_cap('delete_post_meta', $post, '_protected_backcompat') );
+
+			// Register protected key and allow access
+			register_meta( 'post', '_protected_backcompat', array( $this, '_meta_filter' ), array( $this, '_meta_yes_you_can' ) );
+
+			// Test protected key access is now true
+			$this->assertTrue( $admin->has_cap('edit_post_meta',  $post, '_protected_backcompat') );
+			$this->assertTrue( $admin->has_cap('add_post_meta',  $post, '_protected_backcompat') );
+			$this->assertTrue( $admin->has_cap('delete_post_meta',  $post, '_protected_backcompat') );
+
+			// Test non protected key access is true
+			$this->assertTrue( $admin->has_cap('edit_post_meta', $post, 'not_protected_backcompat') );
+			$this->assertTrue( $admin->has_cap('add_post_meta', $post, 'not_protected_backcompat') );
+			$this->assertTrue( $admin->has_cap('delete_post_meta', $post, 'not_protected_backcompat') );
+
+			// Register non protected key and disallow access
+			register_meta( 'post', 'not_protected_backcompat', array( $this, '_meta_filter' ), array( $this, '_meta_no_you_cant' ) );
+
+			// Test non protected key access is now false
+			$this->assertFalse( $admin->has_cap('edit_post_meta',  $post, 'not_protected_backcompat') );
+			$this->assertFalse( $admin->has_cap('add_post_meta',  $post, 'not_protected_backcompat') );
+			$this->assertFalse( $admin->has_cap('delete_post_meta',  $post, 'not_protected_backcompat') );
 		}
 	}