recursive variant container compiler error - c++

Question

I'd like a variant contain copies of objects of its type. Somehow it is not working:
struct value
{
};
class json;
using json = ::boost::variant<
::std::vector<::std::unique_ptr<json> >,
::std::unordered_map<::std::string, ::std::unique_ptr<json> >,
value
>;
json.hpp:116:2: error: conflicting declaration 'using json = '
>;
^
json.hpp:110:7: error: 'class json' has a previous declaration as 'class json'
class json;
I know of 2 workarounds already: ::std::unique_ptr<void>, with a custom deleter, as well as the possibility of using ::boost::any instead of the variant, but are these the only ways? The problem with ::boost::any is that I need to enable RTTI for it to work.

Answer 1

What about:
struct json : ::boost::variant<
::std::vector<::std::unique_ptr<json> >,
::std::unordered_map<::std::string, ::std::unique_ptr<json> >,
value
>
{
using variant::variant;
template <typename U>
json& operator=(U&& u)
{
variant::operator=(::std::forward<U>(u));
return *this;
}
};
That would be the solution, except it doesn't work for me with g++ (constructing json out of vector fails because of ambiguous constructor call). Construction from a const reference to such a vector works, but not not from a non-const reference. I have no idea why. In addition, unique_ptr doesn't work with boost::variant for me because it's uncopyable (shared_ptr does work).