p.cpp

#include <utility>

using namespace std;

auto l = [](char, int) { return 1.0; };

struct foo {
    int operator()(int, char) { return 0; }
};

template <typename C, typename T>
auto prototype_of_impl(T C::* const) -> std::decay_t<T>;
//template <typename C, typename T>
//auto prototype_of_impl(T C::*) -> std::decay_t<T>;

template <typename T>
struct prototype_of {
    using type = decltype(prototype_of_impl(&T::operator()));
};

int main() {
    //using T = prototype_of<decltype(l)>::type;
    using F = prototype_of<foo>::type;
    return 0;
}