Alternative operator attribute is incorrect

X3 compound attribute rules say this:

a: A, b: A --> (a | b): A

However, when I write this simple bit of code (exposition purposes only):

#include <boost/spirit/home/x3.hpp>
int main() {
    using namespace boost::spirit::x3;
    auto const quoted_string = lexeme['"' >> *(char_ - '"') >> '"'];
    auto value = std::string{};
    auto assign_string = [&](auto& ctx){ value = _attr(ctx); };
    auto const any_string = (quoted_string | quoted_string) [assign_string];
    auto val = std::string_view{"\"Test\""};
    phrase_parse(val.begin(), val.end(), any_string, space);
}

I get this error:

error: no match for ‘operator=’ (operand types are ‘std::__cxx11::basic_string<char>’ and ‘boost::variant<boost::detail::variant::over_sequence<boost::mpl::l_item<mpl_::long_<2>, std::__cxx11::basic_string<char>, boost::mpl::l_item<mpl_::long_<1>, std::__cxx11::basic_string<char>, boost::mpl::l_end> > > >’)
   7 |     auto assign_string = [&](auto& ctx){ value = _attr(ctx); };

That implies that the compound attribute rule is actually:

a: A, b: A --> (a | b): variant<A, A>

Why is the attribute a variant instead of a string?

boostorg / spirit

Alternative operator attribute is incorrect #615