Ebuild has new 'driver' USE flag. During world update, the new default is without the 'driver' USE. This results in failure during install phase of emerge. >>> Installing (1 of 1) x11-drivers/nvidia-drivers-340.108::gentoo * checking 184 files for package collisions >>> Merging x11-drivers/nvidia-drivers-340.108 to / sed: can't read /var/tmp/portage/x11-drivers/nvidia-drivers-340.108/image/etc/modprobe.d/nvidia.conf: No such file or directory * ERROR: x11-drivers/nvidia-drivers-340.108::gentoo failed (preinst phase): emerging with USE=driver works fine. If the fix isn't obvious then I can give more complete info. The /etc/modprobe.d dir isn't even created with -driver USE.
Sync and try again, duplicate of bug #735008 fixed a bit under a day ago.