Can't stream std::endl with overloaded operator<<() for std::variant

Question

This answer describes how to stream a standalone std::variant. However, it doesn\'t seem to work when std::variant is stored in a std::unorde

Answer 1

The problem is the std::endl but I am puzzled why your overload is a better match than the one from std::basic_ostream::operator<<, see godbolt live example:

:29:12: note: in instantiation of template class 'std::variant<>' requested here
        << std::endl;
           ^

and removing the std::endl indeed fixes the problem, see it live on Wandbox.

As alfC points out altering your operator to disallow an empty variant does indeed fix the issue, see it live:

template
std::ostream& operator<<(std::ostream& os, const std::variant& v)