I have several using (typedef) declarations for various tcb::span-based types, that I would like to move to a mylib/fwd.h header, which is not supposed to include much, if at all. Thus I'd like to forward-declare tcb::span itself there too, instead of including the header. Is it possible? Beside the main struct, I don't think I need anything else, but just in case, what else could be fwd-decl there too?