X3: Alternative operator doesn't reduce?

X3 compound attribute rules
<https://www.boost.org/doc/libs/1_73_0/libs/spirit/doc/x3/html/spirit_x3/quick_reference/compound_attribute_rules.html>
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::b
asic_string<char>’ and ‘
boost::variant<boost::detail::variant::over_sequence<boo
st::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); };
     |

Why is the attribute a variant instead of a string?

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