Gentoo's Bugzilla –
Attachment 661059 Details for
Bug 743172dev-cpp/eigen-3.3.7 fails to build tests - /var/tmp/portage/dev-cpp/eigen-3.3.7/work/eigen-3.3.7/Eigen/src/Core/MathFunctions.h:623:50: error: invalid use of incomplete type ‘struct Eigen::internal::make_unsigned<long long int>’