Call R function from Rcpp which returns List leads to error during compilation

I'm trying to use the stats::constrOptim function inside of Rcpp Code and I'm only interested in the par object of the returned list.

This is what it looks like in R.

Rcpp::sourceCpp("development/batch_reprex.cpp")

A <- matrix(1:6, ncol = 2)
weights <- c(0.5, 0.5)
pred <- A %*% weights
truth <- c(3, 3, 3)

# Optim constraints:
positivity <- diag(ncol(A))
convexity <- rep(1, ncol(A))
constr_left <- rbind(convexity, positivity)
constr_right <- c(1, rep(0, ncol(A)))

constrOptim(weights,
    subject_function,
    grad = NULL,
    ui = constr_left,
    ci = constr_right - 1e-12, # see https://stackoverflow.com/a/50474095
    method = "Nelder-Mead",
    predictions = A,
    truth = truth
)$par

I just want to do this last call inside of an RCPP function. This is what my batch_reprex.cpp looks like:

// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::depends(RcppProgress)]]
#define RCPP_ARMADILLO_RETURN_VEC_AS_VECTOR
#include <RcppArmadillo.h>
#include <string>
#include <progress.hpp>
using namespace arma;

// [[Rcpp::export]]
double subject_function(const vec &weights,
                        const mat &predictions,
                        const vec &truth)
{
    vec pred = predictions * weights;
    vec loss_vec = arma::pow(pred - truth, 2);
    return arma::mean(loss_vec);
}

// [[Rcpp::export]]
SEXP constr_optim_rcpp(const vec &weights,
                       const mat &predictions,
                       const vec &truth,
                       const mat &ui,
                       const vec &ci)
{
    Rcpp::Environment pkg = Rcpp::Environment::namespace_env("stats");

    Rcpp::Function f = pkg["constrOptim"];

    return f(weights,
             subject_function,
             Rcpp::Named("grad", NULL),
             Rcpp::Named("ui", ui),
             Rcpp::Named("ci", ci),
             Rcpp::Named("method", "Nelder-Mead"),
             Rcpp::Named("predictions", predictions),
             Rcpp::Named("truth", truth));
}

Running sourceCpp returns a lot of error messages including:

/usr/local/lib/R/site-library/Rcpp/include/Rcpp/traits/is_convertible.h:35:12: error: function returning a function
   35 |   static T MakeT() ;
      |            ^~~~~

Thank you so much in advance.

Edit:

I figured out that the problem has something todo with the fact that I'm passing the Rcpp subject_function to constrOptim. When I pass an R function, everything works fine:

// [[Rcpp::export]]
Rcpp::List constr_optim_rcpp(const vec &weights,
                             const mat &predictions,
                             const vec &truth,
                             const mat &ui,
                             const vec &ci)
{
    Rcpp::Environment pkg_base = Rcpp::Environment::namespace_env("base");
    Rcpp::Function f_sum = pkg_base["sum"];
    Rcpp::Environment pkg = Rcpp::Environment::namespace_env("stats");
    Rcpp::Function f = pkg["constrOptim"];
    Rcpp::List res = Rcpp::List::create(f(weights,
                                          f_sum,
                                          R_NilValue,
                                          ui,
                                          ci,
                                          Rcpp::Named("predictions", predictions),
                                          Rcpp::Named("truth", truth)));

    return res;
}