template<class... Ts> struct overloaded : Ts... {
   using Ts::operator()...;

   overloaded(Ts&&... args) : Ts(std::forward<Ts>(args))... {}
};

template<class... Ts> overloaded(Ts&&...) -> overloaded<std::decay_t<Ts>...>;

using var_t = std::variant<int, std::string, std::vector<int>>;

int main() {
   var_t v = 5;

   auto res = std::visit(overloaded(
       [](int i) -> size_t { return i; },
       [](auto& v) -> size_t { return v.size(); }
   ), v);

   std::cout << res << std::endl;
}