Source

File: wp-includes/class-wp-customize-manager.php

public function add_control( $id, $args = array() ) {
		if ( $id instanceof WP_Customize_Control ) {
			$control = $id;
		} else {
			$control = new WP_Customize_Control( $this, $id, $args );
		}

		$this->controls[ $control->id ] = $control;
		return $control;
	}