Why does this get_index implementation fail on VS2017?

问题

Barry gave us this gorgeous get_index for variants:

template <typename> struct tag { };

template <typename T, typename V>
struct get_index;

template <typename T, typename... Ts> 
struct get_index<T, std::variant<Ts...>>
    : std::integral_constant<size_t, std::variant<tag<Ts>...>(tag<T>()).index()>
{ };

To be used as follows:

using V = variant<A, B, C>;
constexpr const size_t N = get_index<B, V>::value;  // 1

It works great in Clang (OSX).

But in Visual Studio 2017 I'm getting the following:

<source>(10): error C2039: 'index': is not a member of 'std::variant<tag<Ts>...>'
<source>(10): note: see declaration of 'std::variant<tag<Ts>...>'
<source>(11): note: see reference to class template instantiation 'get_index<T,std::variant<_Types...>>' being compiled
Compiler returned: 2

I can't see why. Any ideas?

(Full disclosure: in my project I'm actually using mpark::variant because I have been using Xcode 9, which didn't have std::variant. However, you can see from the Godbolt MCVE above that this affects the implementation with std::variant as well. I'm convinced the problem is either in the code above, or in the compiler.)

回答1:

I bet my 2 cents that is a compiler bug.

I see that if I write in main()

std::cout << std::variant<tag<int>, tag<float>>{tag<float>{}}.index() << std::endl;

the compiler doesn't complain.

And doesn't complain also if I write a template function as follows

template <typename T, typename ... Ts>
void foo ()
 { std::cout << std::variant<tag<Ts>...>(tag<T>{}).index() << std::endl; }

and I call it, from main(), with

foo<int, long, int, long long>();

No problem also declaring the following variable in main()

std::integral_constant<std::size_t, std::variant<tag<int>, tag<float>>(tag<float>{}).index()>  ic;

But if I change the get_index specialization as follows (using braces for initialization instead of round parentheses)

template <typename T, typename... Ts> 
struct get_index<T, std::variant<Ts...>>
    : std::integral_constant<std::size_t, std::variant<tag<Ts>...>{tag<T>()}.index()>
 { };

the compiler complain but with a different error

example.cpp

(12): error C2440: 'initializing': cannot convert from 'initializer list' to 'std::variant...>'

(12): note: The target type has no constructors

(13): note: see reference to class template instantiation 'get_index>' being compiled

Compiler returned: 2

Seems that, for reasons I can't understand, the compiler doesn't see std::variant<tag<Ts>...>, inside get_index, as a std::variant with all it's methods.

来源：https://stackoverflow.com/questions/53651609/why-does-this-get-index-implementation-fail-on-vs2017