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

Question

Consider the following code snippet:

struct v : std::variant> { };

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


        

Answer 1

I came across this issue as well recently. I kind of came up with a workaround which basically specialises variant_size and variant_alternative for the class that inherits from the variant..

link on godbolt

Its not pretty and it injects stuff into the std namespace. I'm not a metaprogramming expert (yet!) so its something I hacked together. Maybe someone else can improve on this?

#include 
#include 
#include 
#include 

#include 
#include 



using var = std::variant;

struct myvar : public var {
    using var::var;
    using var::operator=;

};

namespace std{

    template<>
    struct variant_size : variant_size {
    };

    template
    struct variant_alternative :  variant_alternative {
    };
}

int main(){

    constexpr int vs = std::variant_size::value;

    myvar s = std::string{"boo!"}; 
    std::visit([](auto&& e){std::cout << e << "\n";}, s);
    std::cout << vs;
}