Support std::variant

[heretic13]

Hello. I have a problem with using std::variant with boost::format.
Visual Studio 2019 - 16.7.3
boost - 1.74

Sample code:

`
#include < iostream>
#include < variant>
#include <boost/variant.hpp>
#include <boost/format.hpp>

typedef std::variant<uint16_t, std::string> MemCaptureIdType;
//typedef boost::variant<uint16_t, std::string> MemCaptureIdType;

std::ostream& operator<<(std::ostream& os, const MemCaptureIdType& type)
{
return os;
}

int main()
{
auto tmp1 = MemCaptureIdType(0);

auto tmp = boost::format("%1%") % tmp1;

return 0;

}
`

But it works when I use boost::variant.
I think we need to support std::variant.

[xiongyang]

I meet the same problem with std::optional.

gcc (GCC) 13.1.1 20230614 (Red Hat 13.1.1-4)
boost-1.84

here is the code

#include <iostream>
#include <string>
#include <optional>
#include "boost/format.hpp"

template <typename StreamType>
StreamType& operator<<(StreamType& str, const std::optional<double>& id)
{
    return str;
}
int main()
{
        std::optional<double> xxx;
        std::cout << xxx;
        boost::format("%1%") % xxx;
        return 0;
};