Code showing the difference in lookup behaviour for functions and callable objects, in connection to argument-dependent lookup

main.cpp

#include <iostream>
#include <string>

namespace adl_ns {
    struct X{};
    std::string f(const X&){return "f from ADL namespace";}
    std::string g(const X&){return "g from ADL namespace";}
}

namespace using_ns {
    template<class T>
    std::string f(const T&){
        return "f from using namespace";
    }

    struct G {
        constexpr G() = default;
        template<class T>
        constexpr std::string operator()(const T&) {
            return "g from using namespace";
        }
    };
    
    constexpr G g{};
}

int main()
{
    using namespace using_ns;
    adl_ns::X x;
    std::cout << f(x) << std::endl; 
    std::cout << g(x) << std::endl; 
    return 0;
}