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.)
I bet my 2 cents that is a compiler bug.
I see that if I write in
main()
the compiler doesn't complain.
And doesn't complain also if I write a template function as follows
and I call it, from
main()
, withNo problem also declaring the following variable in
main()
But if I change the
get_index
specialization as follows (using braces for initialization instead of round parentheses)the compiler complain but with a different error
Seems that, for reasons I can't understand, the compiler doesn't see
std::variant<tag<Ts>...>
, insideget_index
, as astd::variant
with all it's methods.