boost::get<T>() failed with the x3::forward_ast<> wrapped types

[x0id]

The following code runs as expected:

#include <boost/spirit/home/x3/support/ast/variant.hpp>
#include <iostream>

namespace x3 = boost::spirit::x3;

namespace ast {

struct op_1;

struct expr_1 : x3::variant<
    int,
    x3::forward_ast<op_1>
> {
    using base_type::base_type;
    using base_type::operator=;
};

struct op_1 {
    expr_1 e;
};

struct op_2;

typedef boost::variant<
    int,
    boost::recursive_wrapper<op_2>
> expr_2;

struct op_2 {
    expr_2 e;
};

}

void fun() {
    ast::expr_1 ex1(1);
    ast::expr_2 ex2(2);
    ast::op_1 op1; op1.e = ex1;
    ast::op_2 op2; op2.e = ex2;
    ast::expr_1 ex1a(op1);
    ast::expr_2 ex2a(op2);

    std::cout << boost::get<int>(ex1) << std::endl;
    std::cout << boost::get<x3::forward_ast<ast::op_1>>(&ex1a) << std::endl;
    std::cout << boost::get<int>(ex2) << std::endl;
    std::cout << boost::get<ast::op_2>(&ex2a) << std::endl;
}

int main () {
    fun();
    return 0;
}

But it fails to compile without x3::forward_ast<> wrapper in line 44. In contrast, original boost::variant does not require to specify boost::recursive_wrapper<> in the similar boost::get<>() call (line 46).

Coliru link: https://coliru.stacked-crooked.com/a/f52d436f462056ff

[intractabilis]

boost::get is implemented by Boost Variant library. It knows about boost::recursive_wrapper:

template <class VariantElement, class T>
struct variant_element_functor :
    boost::mpl::or_<
        boost::is_same<VariantElement, T>,
        boost::is_same<VariantElement, boost::recursive_wrapper<T> >,
        boost::is_same<VariantElement, T& >
    >
{};

But it has no clue about boost::spirit::x3::forward_ast. I don't think Boost Spirit can do something about it.