Quickly implemented callable arguments counting (C++20).

argsc.cpp

#include <iostream>
#include <type_traits>
#include <iostream>
#include <cstddef>

#define MAKE_REMOVE_MEM_FN_CV_SPEC(...) \
    template <class_type Class, typename Ret, typename... Args> \
    struct remove_mem_fn_cv<auto (Class::*)(Args...) __VA_ARGS__ -> Ret> { \
        using type = auto (Class::*)(Args...) -> Ret; \
    };

template <typename T>
concept member_function = std::is_member_function_pointer_v<T>;

template <typename T>
concept class_type = std::is_class_v<T>;

template <typename T>
concept functor = requires {
    { class_type<T> };
    { &T::operator() };
};

template <typename>
struct remove_mem_fn_cv {};

MAKE_REMOVE_MEM_FN_CV_SPEC()
MAKE_REMOVE_MEM_FN_CV_SPEC(const)
MAKE_REMOVE_MEM_FN_CV_SPEC(volatile)
MAKE_REMOVE_MEM_FN_CV_SPEC(const volatile)
MAKE_REMOVE_MEM_FN_CV_SPEC(const &)
MAKE_REMOVE_MEM_FN_CV_SPEC(volatile &)
MAKE_REMOVE_MEM_FN_CV_SPEC(const volatile &)
MAKE_REMOVE_MEM_FN_CV_SPEC(const &&)
MAKE_REMOVE_MEM_FN_CV_SPEC(volatile &&)
MAKE_REMOVE_MEM_FN_CV_SPEC(const volatile &&)

namespace detail {

template <typename Func, typename Ret, typename... Args>
constexpr auto mem_fn_argc(auto (Func::*)(Args...) -> Ret) noexcept -> std::size_t {
    return sizeof...(Args);
}

} // namespace detail

constexpr auto argc(member_function auto mem_fn_ptr) noexcept -> std::size_t {
    using stripped_mem_fn = remove_mem_fn_cv<decltype(mem_fn_ptr)>::type;

    return detail::mem_fn_argc(stripped_mem_fn{}); 
}

template <functor Functor>
constexpr auto argc(Functor) noexcept -> std::size_t {
    return argc(&Functor::operator());
}

template <typename R, typename... Ts>
constexpr auto argc(auto (*)(Ts...) -> R) noexcept -> std::size_t {
   return sizeof...(Ts);
}