c++ - Using std::visit on a class inheriting from std::variant - libstdc++ vs libc++

Question

Consider the following code snippet:

struct v : std::variant<int, std::vector<v>> { };

int main()
{
    std::visit([](auto){ }, v{0});
}

• clang++ 7 with -stdlib=libc++ -std=c++2a compiles the code;

• g++ 9 with -std=c++2a fails to compile the code, with the following error:

  /opt/compiler-explorer/gcc-trunk-20180711/include/c++/9.0.0/variant:94:29: error: incomplete type 'std::variant_size' used in nested name specifier

   inline constexpr size_t variant_size_v = variant_size<_Variant>::value;
  
                           ^~~~~~~~~~~~~~
  

live example on godbolt.org

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• Are both implementations conforming to the Standard?

• If not, what implementation is correct here, and why?

Answer 1

[variant.visit] in C++17 doesn't use variant_size_v, but it does in the current working draft as a result of an editorial change. I don't see any indication that LWG reviewed the change before it went in, but it has looked at this part of the standard several times since then and has yet to object to it, so I'm going to postulate that it is in fact required.

Meanwhile, LWG issue 3052, which has been referred to LEWG, would explicitly require std::variant. When that issue is resolved - one way or the other - it should resolve this too.