default controller in sub folder

This what i do

$route['setting'] = "m_admin/setting/sidebar";

m_admin is folder name, Settings is controller name and sidebar is function name

That is what you've done in CI2, you are in the "CodeIgniter 3.x" forum.

@Narf 1- It is working in CI 2.2 (folder/controller).
2- In CI 3.0 ChangeLog Core/Routing section it says - Added support for per-directory default_controller and 404_override classes.
3- In Bug Fixes section - Fixed a bug - :doc:`Routing Library <general/routing>` didn't properly handle default_controller in a subdirectory when a method is also specified.

I think here is a bug or backward incompatibility.

1. Yes, I know it's "working" in CI 2.2.
2. Yes, and it works in a completely different way.
3. Yes, this bug is what has allowed you to say 1. It was a bug that you were able to do that.