is_callable.hpp

template<typename Function, typename Signature>
struct is_callable : std::false_type {
    static_assert(std::is_function<Signature>::value, "Signature must be a function type");
};
 
struct is_callable_impl {
    template<typename Function, typename Return, typename... Args,
             typename R = decltype(std::declval<Function>()(std::declval<Args>()...))>
    static std::is_same<Return, R> test(int);
    template<typename...>
    static std::false_type test(...);
};
 
template<typename Function, typename Return, typename... Args>
struct is_callable<Function, Return(Args...)> : public decltype(is_callable_impl::test<Function, Return, Args...>(0)) {};