Found the following significant warnings:
  bounded_reg.cpp:33:3: warning: 'void arma::set_stream_err2(std::ostream&)' is deprecated (declared at d:/RCompile/CRANpkg/lib/3.7/RcppArmadillo/include/armadillo_bits/debug.hpp:101) [-Wdeprecated-declarations]
  bounded_reg.cpp:33:29: warning: 'void arma::set_stream_err2(std::ostream&)' is deprecated (declared at d:/RCompile/CRANpkg/lib/3.7/RcppArmadillo/include/armadillo_bits/debug.hpp:101) [-Wdeprecated-declarations]
  elastic_net.cpp:35:3: warning: 'void arma::set_stream_err2(std::ostream&)' is deprecated (declared at d:/RCompile/CRANpkg/lib/3.7/RcppArmadillo/include/armadillo_bits/debug.hpp:101) [-Wdeprecated-declarations]
  elastic_net.cpp:35:29: warning: 'void arma::set_stream_err2(std::ostream&)' is deprecated (declared at d:/RCompile/CRANpkg/lib/3.7/RcppArmadillo/include/armadillo_bits/debug.hpp:101) [-Wdeprecated-declarations]
  first_order.cpp:71:55: warning: 'typename arma::enable_if2<(arma::is_arma_type<T1>::value && arma::resolves_to_vector<T1>::yes), const arma::Op<T1, arma::op_sort_vec> >::result arma::sort(const T1&, arma::uword) [with T1 = arma::mtOp<unsigned int, arma::mtOp<unsigned int, arma::eGlue<arma::eOp<arma::Col<double>, arma::eop_abs>, arma::eOp<arma::eOp<arma::Col<double>, arma::eop_abs>, arma::eop_scalar_minus_post>, arma::eglue_plus>, arma::op_rel_lt_post>, arma::op_find_simple>; typename arma::enable_if2<(arma::is_arma_type<T1>::value && arma::resolves_to_vector<T1>::yes), const arma::Op<T1, arma::op_sort_vec> >::result = const arma::Op<arma::mtOp<unsigned int, arma::mtOp<unsigned int, arma::eGlue<arma::eOp<arma::Col<double>, arma::eop_abs>, arma::eOp<arma::eOp<arma::Col<double>, arma::eop_abs>, arma::eop_scalar_minus_post>, arma::eglue_plus>, arma::op_rel_lt_post>, arma::op_find_simple>, arma::op_sort_vec>; arma::uword = unsigned int]' is deprecated (declared at d:/RCompile/CRANpkg/lib/3.7/RcppArmadillo/include/armadillo_bits/fn_sort.hpp:74) [-Wdeprecated-declarations]
  first_order.cpp:147:22: warning: 'typename arma::enable_if2<(arma::is_arma_type<T1>::value && arma::resolves_to_vector<T1>::yes), const arma::Op<T1, arma::op_sort_vec> >::result arma::sort(const T1&, arma::uword) [with T1 = arma::eOp<arma::Col<double>, arma::eop_abs>; typename arma::enable_if2<(arma::is_arma_type<T1>::value && arma::resolves_to_vector<T1>::yes), const arma::Op<T1, arma::op_sort_vec> >::result = const arma::Op<arma::eOp<arma::Col<double>, arma::eop_abs>, arma::op_sort_vec>; arma::uword = unsigned int]' is deprecated (declared at d:/RCompile/CRANpkg/lib/3.7/RcppArmadillo/include/armadillo_bits/fn_sort.hpp:74) [-Wdeprecated-declarations]
  first_order.cpp:153:41: warning: 'typename arma::enable_if2<(arma::is_arma_type<T1>::value && arma::resolves_to_vector<T1>::yes), const arma::Op<T1, arma::op_sort_vec> >::result arma::sort(const T1&, arma::uword) [with T1 = arma::mtOp<unsigned int, arma::mtOp<unsigned int, arma::eGlue<arma::Col<double>, arma::Col<double>, arma::eglue_minus>, arma::op_rel_gt_post>, arma::op_find_simple>; typename arma::enable_if2<(arma::is_arma_type<T1>::value && arma::resolves_to_vector<T1>::yes), const arma::Op<T1, arma::op_sort_vec> >::result = const arma::Op<arma::mtOp<unsigned int, arma::mtOp<unsigned int, arma::eGlue<arma::Col<double>, arma::Col<double>, arma::eglue_minus>, arma::op_rel_gt_post>, arma::op_find_simple>, arma::op_sort_vec>; arma::uword = unsigned int]' is deprecated (declared at d:/RCompile/CRANpkg/lib/3.7/RcppArmadillo/include/armadillo_bits/fn_sort.hpp:74) [-Wdeprecated-declarations]
  first_order.cpp:206:64: warning: 'typename arma::enable_if2<(arma::is_arma_type<T1>::value && arma::resolves_to_vector<T1>::yes), const arma::Op<T1, arma::op_sort_vec> >::result arma::sort(const T1&, arma::uword) [with T1 = arma::mtOp<unsigned int, arma::mtOp<unsigned int, arma::eGlue<arma::eOp<arma::Col<double>, arma::eop_abs>, arma::eOp<arma::eOp<arma::eGlue<arma::eOp<arma::Col<double>, arma::eop_neg>, arma::Col<double>, arma::eglue_plus>, arma::eop_abs>, arma::eop_scalar_minus_post>, arma::eglue_plus>, arma::op_rel_lt_post>, arma::op_find_simple>; typename arma::enable_if2<(arma::is_arma_type<T1>::value && arma::resolves_to_vector<T1>::yes), const arma::Op<T1, arma::op_sort_vec> >::result = const arma::Op<arma::mtOp<unsigned int, arma::mtOp<unsigned int, arma::eGlue<arma::eOp<arma::Col<double>, arma::eop_abs>, arma::eOp<arma::eOp<arma::eGlue<arma::eOp<arma::Col<double>, arma::eop_neg>, arma::Col<double>, arma::eglue_plus>, arma::eop_abs>, arma::eop_scalar_minus_post>, arma::eglue_plus>, arma::op_rel_lt_post>, arma::op_find_simple>, arma::op_sort_vec>; arma::uword = unsigned int]' is deprecated (declared at d:/RCompile/CRANpkg/lib/3.7/RcppArmadillo/include/armadillo_bits/fn_sort.hpp:74) [-Wdeprecated-declarations]
See 'd:/Rcompile/CRANpkg/local/3.7/quadrupen.Rcheck/00install.out' for details.