模板化 using 不能嵌套在 Visual Studio 中

2022-01-12 00:00:00 templates using visual-studio-2017 c++ return-type c++17

这是尽可能简化的，我可以制作一个仍然遇到错误的玩具示例:

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;
}

call_t 来自 Guillaume Racicot 的回答，我想用它来寻找回报类型.但我从 visual-studio-2017 版本 15.6.7:

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

这在 g++ 上运行良好:https://coliru.stacked-crooked.com/a/48b18b66c39486ef 它甚至可以在 visual-studio-2017 如果我没有将一个 using 语句传递给另一个:

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);
}

有什么办法可以解决这个问题吗?

Is there a way I can work around this?

推荐答案

正如@NathanOliver 所提到的，正确的解决方案是升级到 15.9.5 并修复此问题.但除非您可以使用 result_of 或 invoke_result 在 15.6.7 上通过将 call_t 更改为:

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)>;

请注意 result_of 在 c++17 因此，如果您使用/std:c++17"或/std:c++latest"运行，这将不起作用，您将需要使用更方便:

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>;

值得注意的是，Guillaume Racicot's answer 使用了一个优雅的 veradic 模板，它也分别用作:template<typename F, typename...Args>使用 call_t = result_of_t 或 templatefunc 的定义更改为:

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);
}

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