trait to check function accepts certain parameters but not return type

I'm trying to write a type trait in c++11 (msvc2013) that will allow me to check that a function type takes certain parameters. I don't want it to check the return type. I think the idea is basically equivalent to std::is_callable, but I'm interested in knowing what's wrong with my approach, in addition to how to actually solve the problem.

my implementation:

namespace traits
{
    namespace detail
    {
        template <typename T>
        struct is_write_function_impl
        {
            const char* c = nullptr;
            size_t l = 0;

            template<typename U>
            static auto test(U*)->decltype(declval<U>()(c, l), std::true_type);
            template<typename U>
            static auto test(...)->std::false_type;

            using type = decltype(test<T>(0));
        };
    }

    template <typename T>
    struct is_write_function : detail::is_write_function_impl<T>::type {};
}

my test case:

std::ofstream os;
auto valid = std::bind(&std::ofstream::write, &os, 
    std::placeholders::_1, std::placeholders::_2);

// want this to be 'true' but get 'false'
std::cout << traits::is_write_function<decltype(valid)>::value;