C++17 variant<any> inside the class

Question

The following code compiles well:

int main()
{
    variant<any> var;
    var = 5;
    cout << any_cast<int>(get<any>(var)) << endl;
    return 0;
}

But when I'm trying to put variant<any> as a class member

struct MyClass{
    variant<any> var;
};

int main()
{
    MyClass s;
    return 0;
}

It doesn't compile. Am I doing something wrong or it is some bug?

I'm using gcc 7.1.0

In file included from /home/zak/Projects/Anytest/main.cpp:3:0:
/usr/local/gcc-7.1/include/c++/7.1.0/variant: In instantiation of ‘struct std::__detail::__variant::__accepted_index<const std::variant<std::any>&, std::variant<std::any>, void>’:
/usr/local/gcc-7.1/include/c++/7.1.0/variant:911:26:   required from ‘constexpr const size_t std::variant<std::any>::__accepted_index<const std::variant<std::any>&>’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:940:6:   required by substitution of ‘template<class _Tp, class> constexpr std::variant<std::any>::variant(_Tp&&) [with _Tp = const std::variant<std::any>&; <template-parameter-1-2> = <missing>]’
/home/zak/Projects/Anytest/main.cpp:14:13:   required from here
/usr/local/gcc-7.1/include/c++/7.1.0/variant:559:49: error: no matching function for call to ‘std::__detail::__variant::__overload_set<std::any>::_S_fun(const std::variant<std::any>&)’
       decltype(__overload_set<_Types...>::_S_fun(std::declval<_Tp>()),
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note: candidate: static std::integral_constant<long unsigned int, sizeof... (_Rest)> std::__detail::__variant::__overload_set<_First, _Rest ...>::_S_fun(_First) [with _First = std::any; _Rest = {}]
       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
                                                          ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note:   no known conversion for argument 1 from ‘const std::variant<std::any>’ to ‘std::any’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note: candidate: static void std::__detail::__variant::__overload_set<_Types>::_S_fun() [with _Types = {}]
     { static void _S_fun(); };
                   ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note:   candidate expects 0 arguments, 1 provided
/usr/local/gcc-7.1/include/c++/7.1.0/variant:559:49: error: no matching function for call to ‘std::__detail::__variant::__overload_set<std::any>::_S_fun(const std::variant<std::any>&)’
       decltype(__overload_set<_Types...>::_S_fun(std::declval<_Tp>()),
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note: candidate: static std::integral_constant<long unsigned int, sizeof... (_Rest)> std::__detail::__variant::__overload_set<_First, _Rest ...>::_S_fun(_First) [with _First = std::any; _Rest = {}]
       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
                                                          ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note:   no known conversion for argument 1 from ‘const std::variant<std::any>’ to ‘std::any’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note: candidate: static void std::__detail::__variant::__overload_set<_Types>::_S_fun() [with _Types = {}]
     { static void _S_fun(); };
                   ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note:   candidate expects 0 arguments, 1 provided
/usr/local/gcc-7.1/include/c++/7.1.0/variant:559:49: error: no matching function for call to ‘std::__detail::__variant::__overload_set<std::any>::_S_fun(const std::variant<std::any>&)’
       decltype(__overload_set<_Types...>::_S_fun(std::declval<_Tp>()),
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note: candidate: static std::integral_constant<long unsigned int, sizeof... (_Rest)> std::__detail::__variant::__overload_set<_First, _Rest ...>::_S_fun(_First) [with _First = std::any; _Rest = {}]
       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
                                                          ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note:   no known conversion for argument 1 from ‘const std::variant<std::any>’ to ‘std::any’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note: candidate: static void std::__detail::__variant::__overload_set<_Types>::_S_fun() [with _Types = {}]
     { static void _S_fun(); };
                   ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note:   candidate expects 0 arguments, 1 provided
/usr/local/gcc-7.1/include/c++/7.1.0/variant: In instantiation of ‘constexpr const size_t std::__detail::__variant::__accepted_index<const std::variant<std::any>&, std::variant<std::any>, void>::value’:
/usr/local/gcc-7.1/include/c++/7.1.0/variant:911:26:   required from ‘constexpr const size_t std::variant<std::any>::__accepted_index<const std::variant<std::any>&>’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:940:6:   required by substitution of ‘template<class _Tp, class> constexpr std::variant<std::any>::variant(_Tp&&) [with _Tp = const std::variant<std::any>&; <template-parameter-1-2> = <missing>]’
/home/zak/Projects/Anytest/main.cpp:14:13:   required from here
/usr/local/gcc-7.1/include/c++/7.1.0/variant:564:12: error: no matching function for call to ‘std::__detail::__variant::__overload_set<std::any>::_S_fun(const std::variant<std::any>&)’
  - decltype(__overload_set<_Types...>::
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~
      _S_fun(std::declval<_Tp>()))::value;
      ~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note: candidate: static std::integral_constant<long unsigned int, sizeof... (_Rest)> std::__detail::__variant::__overload_set<_First, _Rest ...>::_S_fun(_First) [with _First = std::any; _Rest = {}]
       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
                                                          ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:541:58: note:   no known conversion for argument 1 from ‘const std::variant<std::any>’ to ‘std::any’
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note: candidate: static void std::__detail::__variant::__overload_set<_Types>::_S_fun() [with _Types = {}]
     { static void _S_fun(); };
                   ^~~~~~
/usr/local/gcc-7.1/include/c++/7.1.0/variant:535:19: note:   candidate expects 0 arguments, 1 provided

Answer 1

The problem is with MyClass's implicitly defined copy constructor, when it tries to copy construct the member of type std::variant<std::any>.

To perform overload resolution, the compiler first needs to try to instantiate all of std::variant's constructor templates, with the function argument being a const std::variant<std::any>&¹. The constructor of our interest is this one:

template <class T> constexpr variant(T&& t) noexcept(/*...*/);

It only participates in overload resolution if, among others, the expression FUN(​std​::​forward<T>(t)) is well formed, where FUN is a set of overloaded functions produced according to [variant.ctor]/12.²

In this case, there is only one alternative type (std::any), so there is only one imaginary function FUN, whose signature is FUN(std::any). Now, the compiler needs to decide whether FUN can be called with a const std::variant<std::any>&¹. In this process, the compiler needs to know whether std::any can be constructed with const std::variant<std::any>&¹.

This will trigger the instantiation of std::any's constructor template template<class T> any(T&& value);, which only participates in overload resolution if std::is_­copy_­constructible_­v<VT> is true (VT being std::decay_t<T>, and T being const std::variant<std::any>&).

Now in order to see whether VT (i.e. std::variant<std::any>) is copy constructible, the compiler needs to try to instantiate all of std::variant's constructor templates... and this is where we started, and we are stuck in a loop.

This can explain why we see template<class _Tp, class> constexpr std::variant<std::any>::variant(_Tp&&) and __overload_set<std::any>::_S_fun (which corresponds to the function FUN mentioned above) in the error message, and why we see the same error appearing several times.

It remains a question how GCC breaks from the above loop, and why tweaking the program can stop GCC from reporting the error. Maybe these are indication of some bug.

---

^{1. Strictly speaking, it should be "an lvalue of type const std::variant<std::any>" rather than "a const std::variant<std::any>&".}

^{2. The standard also requires that this constructor shall only participate in overload resolution if is_­same_­v<decay_­t<T>, variant> is false. GCC (libstdc++) chooses to check that later. I don't know whether this is conforming.}