add Chapter18

Chapter18/cib/libs/stdx/function_traits.hpp

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+#pragma once
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/utility.hpp>
+
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+namespace stdx {
+inline namespace v1 {
+namespace detail {
+template <typename...> struct function_traits;
+
+template <typename R, typename... Args>
+struct function_traits<std::function<R(Args...)>> {
+    using return_type = R;
+
+    template <template <typename...> typename List> using args = List<Args...>;
+    template <template <typename...> typename List>
+    using decayed_args = List<std::decay_t<Args>...>;
+    using arity = std::integral_constant<std::size_t, sizeof...(Args)>;
+
+    template <auto N>
+    using nth_arg = boost::mp11::mp_at_c<args<boost::mp11::mp_list>, N>;
+    template <auto N>
+    using decayed_nth_arg =
+        boost::mp11::mp_at_c<decayed_args<boost::mp11::mp_list>, N>;
+};
+} // namespace detail
+
+template <typename F>
+using function_traits =
+    detail::function_traits<decltype(std::function{std::declval<F>()})>;
+
+template <typename F> using return_t = typename function_traits<F>::return_type;
+template <typename F, template <typename...> typename List>
+using args_t = typename function_traits<F>::template args<List>;
+template <typename F, template <typename...> typename List>
+using decayed_args_t = typename function_traits<F>::template decayed_args<List>;
+template <typename F>
+using nongeneric_arity_t = typename function_traits<F>::arity;
+
+template <typename F, auto N>
+using nth_arg_t = typename function_traits<F>::template nth_arg<N>;
+template <typename F, auto N>
+using decayed_nth_arg_t =
+    typename function_traits<F>::template decayed_nth_arg<N>;
+
+namespace detail {
+template <auto> struct any_type {
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    template <typename T> operator T();
+};
+
+template <typename F, std::size_t... Is>
+constexpr auto try_invoke_impl(std::index_sequence<Is...>)
+    -> std::invoke_result_t<F, any_type<Is>...>;
+
+template <typename F, typename N>
+using try_invoke =
+    decltype(try_invoke_impl<F>(std::make_index_sequence<N::value>{}));
+
+template <typename F, typename N>
+using has_arg_count = boost::mp11::mp_valid<try_invoke, F, N>;
+
+template <typename F, typename N> struct generic_arity;
+
+template <typename F, typename N>
+using generic_arity_t = typename generic_arity<F, N>::type;
+
+template <typename F, typename N> struct generic_arity {
+    using type = boost::mp11::mp_eval_if<
+        has_arg_count<F, N>, N, generic_arity_t, F,
+        std::integral_constant<std::size_t, N::value + 1u>>;
+};
+} // namespace detail
+
+template <typename F>
+using arity_t = boost::mp11::mp_eval_or<
+    detail::generic_arity_t<F, std::integral_constant<std::size_t, 0u>>,
+    nongeneric_arity_t, F>;
+template <typename F> constexpr auto arity_v = arity_t<F>::value;
+} // namespace v1
+} // namespace stdx