c – std :: experimental :: is_detected的奇怪MSVC行为

我实现了基于on this article on cppreference.com的std :: experimental :: is_detected(部分代码低于工作repro).

它适用于G和Clang,但导致MSVC的奇怪错误行为:is_detected似乎总是bool_constant< true>!

在这里,您可以使用gcc 5.x查看正确的结果:
@ideone.com

但是使用MSVC 19(随VS2015一起提供),测试总能成功:

Z:\>cl /EHsc test.cxx
....
Z:\>test
true, true

那么,这是编译器中的已知错误吗?是否与表达SFINAE无法正确实现有关?我可以使用任何解决方法来完成这项工作吗?

谢谢!

以下是重现错误的代码的一部分(除了is_detected以外,我省略了接口的其余部分以增加可读性):

#include <iostream>

// void_t: void type alias
template< typename... >
using void_t = void;
//

namespace internal
{
    // Fallback case
    template<   typename D,
                typename Void,
                template< typename... > class Check,
                typename... Args 
            >
    struct detect_impl
    {
        using value_t = std::false_type;
        using type = D;
    };


    // Check succeeded
    template<   typename D,
                template< typename... > class Check,
                typename... Args 
            >
    struct detect_impl
        < D, void_t< Check<Args...> >, Check, Args... >
    {
        using value_t = std::true_type;
        using type = Check<Args...>;
    };
}

// Type representing a missing type.
struct nonesuch
{
    nonesuch() = delete;
    ~nonesuch() = delete;
    nonesuch(nonesuch const&) = delete;
    void operator=(nonesuch const&) = delete;
};


template<   template< typename... > class Check,
            typename... Args
        >
using is_detected = typename internal::detect_impl< nonesuch, void, Check, Args... >::value_t;



// Our test
template< typename T >
using is_addable_impl = decltype( std::declval<T>() + std::declval<T>() );

template< typename T >
using is_addable = is_detected<is_addable_impl, T>;


auto main(int argc, const char* arv[])
    -> int
{
    std::cout   <<  std::boolalpha
                <<  is_addable<int>::value  << ", "
                <<  is_addable<nonesuch>::value << std::endl;
}

编辑:奇怪的是,直接使用void_t惯用法在MSVC上工作:

#include <iostream>
#include <type_traits>

struct X {};

template< typename T, typename = void >
struct is_addable
    : std::false_type
{};

 template< typename T >
 struct is_addable <T, std::void_t<decltype(std::declval<T>() + std::declval<T>())>>
     : std::true_type
 {};

int main()
{
    std::cout   <<  std::boolalpha 
                << is_addable<int>::value   << ", "
                << is_addable<X>::value
                << std::endl;
}

输出:

Z:\>cl /EHsc test.cxx
....
Z:\>test.exe
true, false

解决方法:

这是一个似乎适用于最近的MSVC(使用Visual C 19.00.23720.0测试)的解决方法:

#include <type_traits>

template <typename...>
using void_t = void;

namespace internal
{
    template <typename V, typename D>
    struct detect_impl
    {
        using value_t = V;
        using type = D;
    };

    template <typename D, template <typename...> class Check, typename... Args>
    auto detect_check(char)
        -> detect_impl<std::false_type, D>;

    template <typename D, template <typename...> class Check, typename... Args>
    auto detect_check(int)
        -> decltype(void_t<Check<Args...>>(),
                    detect_impl<std::true_type, Check<Args...>>{});

    template <typename D, typename Void, template <typename...> class Check, typename... Args>
    struct detect : decltype(detect_check<D, Check, Args...>(0)) {};
}

struct nonesuch
{
    nonesuch() = delete;
    ~nonesuch() = delete;
    nonesuch(nonesuch const&) = delete;
    void operator=(nonesuch const&) = delete;
};

template <template< typename... > class Check, typename... Args>
using is_detected = typename internal::detect<nonesuch, void, Check, Args...>::value_t;

(虚拟void参数现在没有使用,只是为了保持实现的其余部分完整.)

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