I'd be interested in an opinion of the dev on what was the hardest part to build. I could imagine reflection features or the incremental type checking would be

I'd be interested in an opinion of the dev on what was the hardest part to build. I could imagine reflection features or the incremental type checking would be hard to do. But I always see these projects (like cling/ROOT from cern) and think that some subproblems are really hard to do if done incrementally with only local knowledge.