faux-function to determine whether a class/struct has a function (optionally checking for a specific type signature)

has_member_function.cpp

#include <type_traits>

//

template <class...>
struct type_pack
{};

//

namespace detail
{
    template <class R, class T>
    auto constexpr match(R T::*)
    -> std::true_type;

    template <class...>
    auto constexpr match(...)
    -> std::false_type;
}

#define has_member_function(...) \
    [] \
    { \
        using type = __VA_ARGS__; \
        HAS_MEMBER_FUNCTION_IMPL_1
#define HAS_MEMBER_FUNCTION_IMPL_1(...) \
        auto constexpr helper = [] \
        <class T, class... Sig> \
        (type_pack<T, Sig...>) \
        -> decltype(detail::match<Sig...>(&T::__VA_ARGS__)) \
        {}; \
        HAS_MEMBER_FUNCTION_IMPL_2
#define HAS_MEMBER_FUNCTION_IMPL_2(...) \
        using input = type_pack<type __VA_OPT__(, __VA_ARGS__)>; \
        return std::is_invocable_r_v<std::true_type, decltype(helper), input>; \
    }()
    
//

template <class...>
struct foo
{
    template <class T>
    auto constexpr func(T) const
    -> T;
};

//

auto main()
-> int
{
    // check for existance of member functions
    //
    // usage:
    //  has_member_function(T[<Ts...>])([template] name[<Us...>])([signature])
    // where 'signature' is:
    //  R([Args...]) [const] [volatile]

    static_assert
    (
        has_member_function
            (foo<int, double>)      // class/struct
            (template func<int>)    // member function name
            (int(int) const)        // [signature]
    );
}