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

Calculating dependencies ... done!
Dependency resolution took 0.87 s (backtrack: 0/20).


!!! The ebuild selected to satisfy "x11-misc/lightdm" has unmet requirements.
- x11-misc/lightdm-1.32.0-r5::gentoo USE="X gnome gtk -audit -elogind -introspection -non-root (-systemd) -vala"

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

  The above constraints are a subset of the following complete expression:
    exactly-one-of ( elogind systemd ) vala? ( introspection )

(dependency required by "x11-misc/light-locker-1.9.0-r2::gentoo" [ebuild])
(dependency required by "=x11-misc/light-locker-1.9.0-r2" [argument])