diff -r 3d4e9c994f10 -r a86126ab1dd4 wp/wp-includes/class-wp-taxonomy.php
--- a/wp/wp-includes/class-wp-taxonomy.php	Tue Oct 22 16:11:46 2019 +0200
+++ b/wp/wp-includes/class-wp-taxonomy.php	Tue Dec 15 13:49:49 2020 +0100
@@ -30,12 +30,17 @@
 	public $label;
 
 	/**
-	 * An array of labels for this taxonomy.
+	 * Labels object for this taxonomy.
+	 *
+	 * If not set, tag labels are inherited for non-hierarchical types
+	 * and category labels for hierarchical ones.
+	 *
+	 * @see get_taxonomy_labels()
 	 *
 	 * @since 4.7.0
 	 * @var object
 	 */
-	public $labels = array();
+	public $labels;
 
 	/**
 	 * A short descriptive summary of what the taxonomy is for.
@@ -205,6 +210,25 @@
 	public $rest_controller_class;
 
 	/**
+	 * The default term name for this taxonomy. If you pass an array you have
+	 * to set 'name' and optionally 'slug' and 'description'.
+	 *
+	 * @since 5.5.0
+	 * @var array|string
+	 */
+	public $default_term;
+
+	/**
+	 * The controller instance for this taxonomy's REST API endpoints.
+	 *
+	 * Lazily computed. Should be accessed using {@see WP_Taxonomy::get_rest_controller()}.
+	 *
+	 * @since 5.5.0
+	 * @var WP_REST_Controller $rest_controller
+	 */
+	public $rest_controller;
+
+	/**
 	 * Whether it is a built-in taxonomy.
 	 *
 	 * @since 4.7.0
@@ -217,7 +241,7 @@
 	 *
 	 * @since 4.7.0
 	 *
-	 * @global WP $wp WP instance.
+	 * @global WP $wp Current WordPress environment instance.
 	 *
 	 * @param string       $taxonomy    Taxonomy key, must not exceed 32 characters.
 	 * @param array|string $object_type Name of the object type for the taxonomy object.
@@ -273,12 +297,13 @@
 			'show_in_rest'          => false,
 			'rest_base'             => false,
 			'rest_controller_class' => false,
+			'default_term'          => null,
 			'_builtin'              => false,
 		);
 
 		$args = array_merge( $defaults, $args );
 
-		// If not set, default to the setting for public.
+		// If not set, default to the setting for 'public'.
 		if ( null === $args['publicly_queryable'] ) {
 			$args['publicly_queryable'] = $args['public'];
 		}
@@ -290,11 +315,11 @@
 				$args['query_var'] = sanitize_title_with_dashes( $args['query_var'] );
 			}
 		} else {
-			// Force query_var to false for non-public taxonomies.
+			// Force 'query_var' to false for non-public taxonomies.
 			$args['query_var'] = false;
 		}
 
-		if ( false !== $args['rewrite'] && ( is_admin() || '' != get_option( 'permalink_structure' ) ) ) {
+		if ( false !== $args['rewrite'] && ( is_admin() || get_option( 'permalink_structure' ) ) ) {
 			$args['rewrite'] = wp_parse_args(
 				$args['rewrite'],
 				array(
@@ -309,27 +334,27 @@
 			}
 		}
 
-		// If not set, default to the setting for public.
+		// If not set, default to the setting for 'public'.
 		if ( null === $args['show_ui'] ) {
 			$args['show_ui'] = $args['public'];
 		}
 
-		// If not set, default to the setting for show_ui.
+		// If not set, default to the setting for 'show_ui'.
 		if ( null === $args['show_in_menu'] || ! $args['show_ui'] ) {
 			$args['show_in_menu'] = $args['show_ui'];
 		}
 
-		// If not set, default to the setting for public.
+		// If not set, default to the setting for 'public'.
 		if ( null === $args['show_in_nav_menus'] ) {
 			$args['show_in_nav_menus'] = $args['public'];
 		}
 
-		// If not set, default to the setting for show_ui.
+		// If not set, default to the setting for 'show_ui'.
 		if ( null === $args['show_tagcloud'] ) {
 			$args['show_tagcloud'] = $args['show_ui'];
 		}
 
-		// If not set, default to the setting for show_ui.
+		// If not set, default to the setting for 'show_ui'.
 		if ( null === $args['show_in_quick_edit'] ) {
 			$args['show_in_quick_edit'] = $args['show_ui'];
 		}
@@ -346,7 +371,7 @@
 
 		$args['object_type'] = array_unique( (array) $object_type );
 
-		// If not set, use the default meta box
+		// If not set, use the default meta box.
 		if ( null === $args['meta_box_cb'] ) {
 			if ( $args['hierarchical'] ) {
 				$args['meta_box_cb'] = 'post_categories_meta_box';
@@ -371,6 +396,21 @@
 			}
 		}
 
+		// Default taxonomy term.
+		if ( ! empty( $args['default_term'] ) ) {
+			if ( ! is_array( $args['default_term'] ) ) {
+				$args['default_term'] = array( 'name' => $args['default_term'] );
+			}
+			$args['default_term'] = wp_parse_args(
+				$args['default_term'],
+				array(
+					'name'        => '',
+					'slug'        => '',
+					'description' => '',
+				)
+			);
+		}
+
 		foreach ( $args as $property_name => $property_value ) {
 			$this->$property_name = $property_value;
 		}
@@ -395,7 +435,7 @@
 			$wp->add_query_var( $this->query_var );
 		}
 
-		if ( false !== $this->rewrite && ( is_admin() || '' != get_option( 'permalink_structure' ) ) ) {
+		if ( false !== $this->rewrite && ( is_admin() || get_option( 'permalink_structure' ) ) ) {
 			if ( $this->hierarchical && $this->rewrite['hierarchical'] ) {
 				$tag = '(.+?)';
 			} else {
@@ -447,4 +487,40 @@
 	public function remove_hooks() {
 		remove_filter( 'wp_ajax_add-' . $this->name, '_wp_ajax_add_hierarchical_term' );
 	}
+
+	/**
+	 * Gets the REST API controller for this taxonomy.
+	 *
+	 * Will only instantiate the controller class once per request.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @return WP_REST_Controller|null The controller instance, or null if the taxonomy
+	 *                                 is set not to show in rest.
+	 */
+	public function get_rest_controller() {
+		if ( ! $this->show_in_rest ) {
+			return null;
+		}
+
+		$class = $this->rest_controller_class ? $this->rest_controller_class : WP_REST_Terms_Controller::class;
+
+		if ( ! class_exists( $class ) ) {
+			return null;
+		}
+
+		if ( ! is_subclass_of( $class, WP_REST_Controller::class ) ) {
+			return null;
+		}
+
+		if ( ! $this->rest_controller ) {
+			$this->rest_controller = new $class( $this->name );
+		}
+
+		if ( ! ( $this->rest_controller instanceof $class ) ) {
+			return null;
+		}
+
+		return $this->rest_controller;
+	}
 }