It's this: https://github.com/p-ranav/structopt/blob/master/include/structopt/third_party/visit_struct/visit_struct.hpp https://github.com/p-ranav/structopt/blo

It's this: https://github.com/p-ranav/structopt/blob/master/include/structopt/third_party/visit_struct/visit_struct.hpp https://github.com/p-ranav/structopt/blob/master/include/str... mashed into a single header with some other stuff by a build tool, I think. That looks like something that was proposed for inclusion in boost a long time ago. I think it eventually got accepted after some renaming :) The amount of preprocessor magic in that file makes my head hurt.