get for variants fail under clang++ but not g++

Question

The following code:

variant x = \"abc\";
cout << get(x) << \"\\n\";

works fine under g++ (version 7

Answer 1

This is caused by clang bug 31852 (and also 33222), whose reproduction courtesy of Jonathan Wakely should look very relevant:

template auto get(V&) { }

template
class variant
{
    template friend auto get(V&);
};

int main()
{
  variant v{};
  get(v); // error: ambiguous 
}

clang doesn't properly recognize friend declarations that have placeholder types. Which is exactly how libstdc++ implements std::get:

// Returns the typed storage for __v.
template
constexpr decltype(auto) __get(_Variant&& __v)
{
    return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
}

this accesses a private member of variant, but this function is properly declared a friend:

template
friend constexpr decltype(auto) __detail::__variant::__get(_Vp&& __v);

libstdc++'s implementation is valid, clang just doesn't think __get is a friend.