Source

File: wp-admin/includes/class-wp-users-list-table.php

public function current_action() {
		if ( ( isset( $_REQUEST['changeit'] ) || isset( $_REQUEST['changeit2'] ) ) &&
			( ! empty( $_REQUEST['new_role'] ) || ! empty( $_REQUEST['new_role2'] ) ) ) {
			return 'promote';
		}

		return parent::current_action();
	}