Add project files.

include/cliforge/function_traits.hpp

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+#pragma once
+//
+// function_traits.hpp
+//
+// Deduces the return type and argument-type pack of "anything callable":
+// plain free-function pointers, non-generic lambdas (captureless or
+// capturing), and std::function instances. This is what lets
+// Command::action() figure out, at compile time, exactly which C++ types
+// it needs to convert parsed CLI tokens into -- without the user ever
+// having to spell the types out twice.
+//
+#include <cstddef>
+#include <functional>
+#include <tuple>
+
+namespace cliforge::detail {
+
+template <typename T>
+struct function_traits;
+
+// Plain free function: void foo(std::string, int)
+template <typename Ret, typename... Args>
+struct function_traits<Ret (*)(Args...)> {
+    using return_type = Ret;
+    static constexpr std::size_t arity = sizeof...(Args);
+    template <std::size_t I>
+    using arg = std::tuple_element_t<I, std::tuple<Args...>>;
+    using args_tuple = std::tuple<Args...>;
+};
+
+// std::function<void(std::string, int)>
+template <typename Ret, typename... Args>
+struct function_traits<std::function<Ret(Args...)>> {
+    using return_type = Ret;
+    static constexpr std::size_t arity = sizeof...(Args);
+    template <std::size_t I>
+    using arg = std::tuple_element_t<I, std::tuple<Args...>>;
+    using args_tuple = std::tuple<Args...>;
+};
+
+// Pointer-to-member-function of operator(), const and non-const, used to
+// peel apart lambdas (which are just anonymous functors under the hood).
+template <typename C, typename Ret, typename... Args>
+struct function_traits<Ret (C::*)(Args...) const> {
+    using return_type = Ret;
+    static constexpr std::size_t arity = sizeof...(Args);
+    template <std::size_t I>
+    using arg = std::tuple_element_t<I, std::tuple<Args...>>;
+    using args_tuple = std::tuple<Args...>;
+};
+
+template <typename C, typename Ret, typename... Args>
+struct function_traits<Ret (C::*)(Args...)> {
+    using return_type = Ret;
+    static constexpr std::size_t arity = sizeof...(Args);
+    template <std::size_t I>
+    using arg = std::tuple_element_t<I, std::tuple<Args...>>;
+    using args_tuple = std::tuple<Args...>;
+};
+
+// Fallback: any other callable (lambda, functor) -- peel off operator().
+template <typename F>
+struct function_traits : function_traits<decltype(&F::operator())> {};
+
+}  // namespace cliforge::detail