code    Error code.
	 * @param string     $message Error message.
	 * @param mixed      $data    Optional. Error data.
	 */
	public function add( $code, $message, $data = '' ) {
		$this->errors[ $code ][] = $message;

		if ( ! empty( $data ) ) {
			$this->add_data( $data, $code );
		}

		/**
		 * Fires when an error is added to a WP_Error object.
		 *
		 * @since 5.6.0
		 *
		 * @param string|int $code     Error code.
		 * @param string     $message  Error message.
		 * @param mixed      $data     Error data. Might be empty.
		 * @param WP_Error   $wp_error The WP_Error object.
		 */
		do_action( 'wp_error_added', $code, $message, $data, $this );
	}

	/**
	 * Adds data to an error with the given code.
	 *
	 * @since 2.1.0
	 * @since 5.6.0 Errors can now contain more than one item of error data. {@see WP_Error::$additional_data}.
	 *
	 * @param mixed      $data Error data.
	 * @param string|int $code Error code.
	 */
	public function add_data( $data, $code = '' ) {
		if ( empty( $code ) ) {
			$code = $this->get_error_code();
		}

		if ( isset( $this->error_data[ $code ] ) ) {
			$this->additional_data[ $code ][] = $this->error_data[ $code ];
		}

		$this->error_data[ $code ] = $data;
	}

	/**
	 * Retrieves all error data for an error code in the order in which the data was added.
	 *
	 * @since 5.6.0
	 *
	 * @param string|int $code Error code.
	 * @return mixed[] Array of error data, if it exists.
	 */
	public function get_all_error_data( $code = '' ) {
		if ( empty( $code ) ) {
			$code = $this->get_error_code();
		}

		$data = array();

		if ( isset( $this->additional_data[ $code ] ) ) {
			$data = $this->additional_data[ $code ];
		}

		if ( isset( $this->error_data[ $code ] ) ) {
			$data[] = $this->error_data[ $code ];
		}

		return $data;
	}

	/**
	 * Removes the specified error.
	 *
	 * This function removes all error messages associated with the specified
	 * error code, along with any error data for that code.
	 *
	 * @since 4.1.0
	 *
	 * @param string|int $code Error code.
	 */
	public function remove( $code ) {
		unset( $this->errors[ $code ] );
		unset( $this->error_data[ $code ] );
		unset( $this->additional_data[ $code ] );
	}

	/**
	 * Merges the errors in the given error object into this one.
	 *
	 * @since 5.6.0
	 *
	 * @param WP_Error $error Error object to merge.
	 */
	public function merge_from( WP_Error $error ) {
		static::copy_errors( $error, $this );
	}

	/**
	 * Exports the errors in this object into the given one.
	 *
	 * @since 5.6.0
	 *
	 * @param WP_Error $error Error object to export into.
	 */
	public function export_to( WP_Error $error ) {
		static::copy_errors( $this, $error );
	}

	/**
	 * Copies errors from one WP_Error instance to another.
	 *
	 * @since 5.6.0
	 *
	 * @param WP_Error $from The WP_Error to copy from.
	 * @param WP_Error $to   The WP_Error to copy to.
	 */
	protected static function copy_errors( WP_Error $from, WP_Error $to ) {
		foreach ( $from->get_error_codes() as $code ) {
			foreach ( $from->get_error_messages( $code ) as $error_message ) {
				$to->add( $code, $error_message );
			}

			foreach ( $from->get_all_error_data( $code ) as $data ) {
				$to->add_data( $data, $code );
			}
		}
	}
}