std::bind_front<42>() seems to be currently well-formed, but the result isn't invocable with any arguments. Given we are already detecting types and values of the NTTP for new overloads of std::bind_front, std::bind_back, and std::not_fn, it might be better to diagnose such cases when NTTP is of a scalar type.

Note that we might need to separately handle NTTP of scalar and class types for these functions when CWG2459 is not resolved, because it might be necessary to avoid copying a template parameter object.