Source

File: wp-includes/class-wp-widget.php

public function _register_one( $number = -1 ) {
		wp_register_sidebar_widget( $this->id, $this->name, $this->_get_display_callback(), $this->widget_options, array( 'number' => $number ) );
		_register_widget_update_callback( $this->id_base, $this->_get_update_callback(), $this->control_options, array( 'number' => -1 ) );
		_register_widget_form_callback( $this->id, $this->name, $this->_get_form_callback(), $this->control_options, array( 'number' => $number ) );
	}