These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 0.89 s (backtrack: 0/20).


!!! The ebuild selected to satisfy ">=gnome-base/gnome-session-45.0" has unmet requirements.
- gnome-base/gnome-session-50.1::gentoo USE="-doc -elogind (-systemd)"

  The following REQUIRED_USE flag constraints are unsatisfied:
    exactly-one-of ( elogind systemd )

(dependency required by "gnome-base/gnome-core-apps-45.2::gentoo" [ebuild])
(dependency required by "=gnome-base/gnome-core-apps-45.2" [argument])