This should be < not <= since size() of 7 has elements of 0,1,2,3,4,5,6. I think in fixing that you can remove your most recent change of || pose_id == global_plan_.poses.size(), yeah? Or if not, please explain why that's there. If this is actually the last point, then why bother detecting it?

|| pose_id == global_plan_.poses.size()

This is very important. There maybe a scenario, where there is no cusp at all in the path (easiest example will be: imagine a robot driving straight to a goal). In that scenario, we need the distance of the goal from the robot's position, so that here, we don't satisfy this condition. Because the return value from this function where the code lies will be 0 and the robot will not move at all,, because the marked condition will satisfy and the lookahead_distance will be assigned 0.0.

Well the <= should definitely be <, if we can agree on that we can separate the topics

I see. Honestly you bring up a good point, 0.0 is a bad return value. How about change return 0.0 to return std::numeric_limits<double>::max(). Then it should always be > lookahead_dist for the check later. Then you can remove that check in the loop and when the for exits from no cusp it returns max.

Well the <= should definitely be <, if we can agree on that we can separate the topics

Agreed, I think it makes sense.

Honestly you bring up a good point, 0.0 is a bad return value. How about change return 0.0 to return std::numeric_limits::max().

This is a valid point, which I would definitely change!