compilation warnings on ubuntu with gcc

[tdhock]

hi @yixuan I just installed RSpectra from the source code on CRAN, and I see the following warnings from gcc-7.5.0. I also get these warnings using gcc-8.4.0.
RSpectra-warnings.txt

There are a lot of warnings, here are just a few. They seem to be coming from RcppEigen so you may consider asking the RcppEigen (or Eigen) devs to fix these.

/home/tdhock/lib/R/library/RcppEigen/include/Eigen/src/Core/arch/SSE/Complex.h:232:1: note: in expansion of macro ‘EIGEN_MAKE_CONJ_HELPER_CPLX_REAL’
 EIGEN_MAKE_CONJ_HELPER_CPLX_REAL(Packet2cf,Packet4f)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/tdhock/lib/R/library/RcppEigen/include/Eigen/src/Core/arch/Default/ConjHelper.h:22:70: warning: ignoring attributes on template argument ‘Eigen::internal::Packet4f {aka __vector(4) float}’ [-Wignored-attributes]
   template<> struct conj_helper<PACKET_CPLX, PACKET_REAL, false,false> {                                          \
                                                                      ^
/home/tdhock/lib/R/library/RcppEigen/include/Eigen/src/Core/arch/SSE/Complex.h:232:1: note: in expansion of macro ‘EIGEN_MAKE_CONJ_HELPER_CPLX_REAL’
 EIGEN_MAKE_CONJ_HELPER_CPLX_REAL(Packet2cf,Packet4f)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/tdhock/lib/R/library/RcppEigen/include/Eigen/src/Core/arch/Default/ConjHelper.h:15:70: warning: ignoring attributes on template argument ‘Eigen::internal::Packet2d {aka __vector(2) double}’ [-Wignored-attributes]
   template<> struct conj_helper<PACKET_REAL, PACKET_CPLX, false,false> {                                          \
                                                                      ^
/home/tdhock/lib/R/library/RcppEigen/include/Eigen/src/Core/arch/SSE/Complex.h:417:1: note: in expansion of macro ‘EIGEN_MAKE_CONJ_HELPER_CPLX_REAL’
 EIGEN_MAKE_CONJ_HELPER_CPLX_REAL(Packet1cd,Packet2d)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/tdhock/lib/R/library/RcppEigen/include/Eigen/src/Core/arch/Default/ConjHelper.h:22:70: warning: ignoring attributes on template argument ‘Eigen::internal::Packet2d {aka __vector(2) double}’ [-Wignored-attributes]
   template<> struct conj_helper<PACKET_CPLX, PACKET_REAL, false,false> {                                          \
                                                                      ^

to be clear, RSpectra does successfully install, but typically it is good to edit your code to suppress such warnings.