Xtensor operator<<overload

Question

I want to keep variant inside xtensor, but when I try to print the tensor the overload for operator<< is not picked up.

#include <iostream>
#include <xtensor/xarray.hpp>
#include <xtensor/xio.hpp>
#include <xtensor/xview.hpp>
#include <xtensor/xadapt.hpp>
#include <variant>

template<typename T, typename... Ts>
std::ostream &operator<<(std::ostream &os, const std::variant<T, Ts...>& v) {
    std::visit([&os](auto &&arg) {
        os << arg;
    }, v);
    return os;
}

void foo() {
    xt::xtensor<std::variant<double, std::string>, 2>::shape_type shape = {1,1};
    xt::xtensor<std::variant<double, std::string>, 2> ret(shape);

    std::variant<double, std::string> a = 2.5;

    ret.at(0, 0) =a;

    std::cout << a << std::endl; //works
    std::cout << ret << std::endl; //doesn`t work
}