Hi,

I would like to help to migrate Nav2 to BT.CPP 4.x before the release of ROS J.

I am playing with it and... it compiles on my machine (that is a start!)

Few initial thoughts:

1. Very stupid question first: does the main compile only in Rolling?

2. Some methods in BT.CPP are now [[nodiscard]] (modern c++, yay) . The changes to address that can be merged earlier and are backward compatible with BT.CPP 3.8 : [behavior_tree] support both ports and blackboard #4060

3. I see that Nav2 broadly use "hard-coded" access to the blackboard, basically skipping the entire InputPort and OutputPort, Subtree remapping, mechanism (that exists for a reason). I would love to discuss in the next community meeting how to address this bad practice. I will try to write a small article about why and how that can be solved the next week.

4. I need to improve the automated XML converter to upgrade from version 3.X tyo 4.X here. Hopefully, that should takle care of most of the migration pain for many users.

Thoughts?