crow::black_magic::const_str doesn't seem to be able to be used with const std::string types

#include "crow/amalgamate/crow_all.h"

struct route
{
    // the relative URL used
    std::string URL;

    // the HTTP method to use
    std::string method;

    // a function pointer expected to process the route entry
    void (*route_handler)(crow::SimpleApp &, route &);
};

// -------

void index_handler( crow::SimpleApp &app, route route_specs )
{
    const std::string url =   (crow::black_magic::const_str) route_specs.URL;
    std::string method = (crow::black_magic::const_str) route_specs.method;

    // TODO need to figure out how this ""_method operator actually works to interpolate route_specs.method
    CROW_ROUTE(app, url).methods("GET"_method)
            ( [] () {
                // expects (route)index
                return "API is online.";
            });
}

void add_two_numbers( crow::SimpleApp &app, route route_specs )
{
    std::string url =   (crow::black_magic::const_str) route_specs.URL;
    std::string method = (crow::black_magic::const_str) route_specs.method;

    // TODO need to figure out how this ""_method operator actually works to interpolate route_specs.method
    CROW_ROUTE(app, url).methods("POST"_method)
            ( [] (const crow::request& req) {
                auto x = crow::json::load(req.body);
                auto y = crow::json::dump(x);
                if (!x)
                    return crow::response(400);
                int sum = x["a"].i()+x["b"].i();
                std::ostringstream os;
                os << sum;
                return crow::response{os.str()};
            } );
}

// -------

class RestAPI
{
    public:
        RestAPI( crow::SimpleApp &app, std::list<route> shape );
};


RestAPI::RestAPI( crow::SimpleApp &app, std::list<route> shape ) {
    // executes 3rd member of all members of (APIShape)shape with its parent struct as its first argument?
    // for entry in shape run entry.handler
    std::list<route>::iterator it;
    for (it = shape.begin(); it != shape.end(); ++it )
    {
        it->route_handler( app, *it );
    }
}
// ------

int main(int argc, char *argv[])
{
    crow::SimpleApp app;

    std::list<route> apiShape;

    route index =       { "/", "GET", &index_handler };
    route add_nums  =   { "/add_two_numbers/<int>/<int>", "GET", &add_two_numbers };


    
    apiShape.push_back( index );
    apiShape.push_back( add_nums );

    RestAPI restAPI( app, apiShape );

/*
    CROW_ROUTE(app, "/").methods("GET"_method)
            ( [] () {
                return "Hello world!";
            });


    CROW_ROUTE(app,"/hello/<int>/<int>").methods("GET"_method)
            ( [] (int count, int count2) {
                if (count > 100)
                    return crow::response(400);
                std::ostringstream os;
                os << count << " bottles of beer!" << "and " << count2;
                return crow::response(os.str());
            } );

    CROW_ROUTE(app, "/add_json").methods("POST"_method)
            ( [] (const crow::request& req) {
                auto x = crow::json::load(req.body);
                auto y = crow::json::dump(x);
                if (!x)
                    return crow::response(400);
                int sum = x["a"].i()+x["b"].i();
                std::ostringstream os;
                // os << sum;
                os << y;
                return crow::response{os.str()};
            } );
*/
    app.port(18080).run();

}