hello,
i repaired this condition by compiling and installing the source from portage distfiles.
cd ~/devel
tar xvf /usr/portage/distfiles/ncurses-6.0.tar.gz
cd ncurses-6.0
./configure
make
sudo make install ( this put ncurses in /usr/local )
emerge ncurses ( result was sys-libs/ncurses-6.0-r1 installed ok )
sudo make uninstall
regards