y combinator for static lambda without deducing this and std::function

y_combinator.cc

#include <type_traits>

template<typename, typename>
struct YComb;

template<typename F, typename Ret, typename...Args>
struct YComb<F, Ret(Args...)>{
    constexpr YComb(F&& f){
    }

    static constexpr Ret operator()(Args... args) {
        return self(args...);
    }

    inline static constexpr Ret(*self)(Args...) = static_cast<Ret(*)(Args...)>(&F::template operator()<YComb::operator()>);
};

template<typename F>
YComb(F&&) -> YComb<F, decltype(F::template operator()<&decltype([] static {})::operator()>)>;


template<typename F>
concept Self = std::is_function_v<std::remove_pointer_t<F>>;

inline static YComb fib = []<Self auto self>(unsigned x) static -> unsigned {
    if (x < 2) return x;
    return self(x - 1) + self(x - 2);
};

int main()
{
    static_assert(fib(8) == 21);
}