What’s with all the vscan, basic_args and arg_store stuff?

This approach is borrowed (cough stolen cough) from fmtlib, for the same reason it’s in there as well. Consider this piece of code:

int i;
std::string str;

scn::scan(range, scn::default_tag, i, str);
scn::scan(range, scn::default_tag, str, i);

If the arguments were not type-erased, almost all of the internals would have to be instantiated for every given combination of argument types.