Source

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

public function add_setting( $id, $args = array() ) {
		if ( $id instanceof WP_Customize_Setting ) {
			$setting = $id;
		} else {
			$class = 'WP_Customize_Setting';

			/** This filter is documented in wp-includes/class-wp-customize-manager.php */
			$args = apply_filters( 'customize_dynamic_setting_args', $args, $id );

			/** This filter is documented in wp-includes/class-wp-customize-manager.php */
			$class = apply_filters( 'customize_dynamic_setting_class', $class, $id, $args );

			$setting = new $class( $this, $id, $args );
		}

		$this->settings[ $setting->id ] = $setting;
		return $setting;
	}