c++ - Templated usings Can't be Nested in Visual Studio

Question

This is about as simplified as I could make a toy example that still hit the bug:

struct Vector3f64 {
    double x;
    double y;
    double z;
};

struct Vector3f32 {
    float x;
    float y;
    float z;
};

// I use this to select their element type in functions:
template <typename T>
using param_vector = std::conditional_t<std::is_same_v<std::remove_const_t<std::remove_reference_t<T>>, Vector3f64>, double, float>;

// This is the function I want to pull the return type from:
template <typename T>
T VectorVolume(const T x, const T y, const T z) {
    return x * x + y * y + z * z;
}

template<typename F, typename T>
using call_t = decltype(std::declval<F>()(std::declval<T>(), std::declval<T>(), std::declval<T>()));

// This function fails to compile:
template <typename T>
call_t<decltype(&VectorVolume<param_vector<T>>), param_vector<T>> func(const T& param) {
    return VectorVolume(param.x, param.y, param.z);
}

int main() {
    const Vector3f64 foo{ 10.0, 10.0, 10.0 };

    std::cout << func(foo) << std::endl;
}

The call_t is from Guillaume Racicot's answer which I wanted to use to find a return type. But I get this error from visual-studio-2017 version 15.6.7:

error C2064: term does not evaluate to a function taking 3 arguments<br>
note: see reference to alias template instantiation 'call_t<unknown-type,double>' being compiled
note: see reference to function template instantiation 'unknown-type func(const T &)' being compiled

This works fine on g++: https://coliru.stacked-crooked.com/a/48b18b66c39486ef It'll even work fine on visual-studio-2017 if I don't pass one using statement to another:

template <typename T>
call_t<decltype(&VectorVolume<param_vector<T>>), double> func(const T& param) {
    return VectorVolume(param.x, param.y, param.z);
}

Is there a way I can work around this?

Answer 1

As mentioned by @NathanOliver the right solution is to upgrade to 15.9.5 where this is fixed. But barring that you can use result_of or invoke_result to solve that on 15.6.7 by changing call_t to:

template<typename F, typename T>
using call_t = result_of_t<F&&(T, T, T)>;

Note that result_of is deprecated in c++17 so if you are running with "/std:c++17" or "/std:c++latest" this won't work, you'll need to use the more convenient:

template<typename F, typename T>
using call_t = invoke_result_t<F, T, T, T>;

It's worth noting that Guillaume Racicot's answer used an elegant veradic template, which also works respectively as: template <typename F, typename... Args> using call_t = result_of_t<F&&(Args&&...)> or template<typename F, typename... Args> using call_t = invoke_result_t<F, Args...>; if you change your definition of func to:

template <typename T>
call_t<decltype(&VectorVolume<param_vector<T>>), param_vector<T>, param_vector<T>, param_vector<T>> func(const T& param) {
    return VectorVolume(param.x, param.y, param.z);
}