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

Calculating dependencies ... done!
Dependency resolution took 1.09 s (backtrack: 0/20).


The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by gnome-base/gnome-shell-50.5::gentoo
# required by gnome-extra/gnome-browser-connector-42.1-r1::gentoo
# required by =gnome-extra/gnome-browser-connector-42.1-r1 (argument)
>=app-crypt/gcr-4.4.1 introspection
# required by gnome-base/gnome-shell-50.5::gentoo
# required by gnome-extra/gnome-browser-connector-42.1-r1::gentoo
# required by =gnome-extra/gnome-browser-connector-42.1-r1 (argument)
>=gui-libs/gtk-4.22.5-r1 wayland introspection
# required by gnome-base/gnome-shell-50.5::gentoo
# required by gnome-extra/gnome-browser-connector-42.1-r1::gentoo
# required by =gnome-extra/gnome-browser-connector-42.1-r1 (argument)
>=x11-wm/mutter-50.5 introspection

!!! The ebuild selected to satisfy ">=x11-wm/mutter-50.0:0/18[introspection,test?]" has unmet requirements.
- x11-wm/mutter-50.5::gentoo USE="(xwayland) -bash-completion -debug -devkit -elogind -gnome -gtk-doc -introspection -screencast (-selinux) -sysprof (-systemd) -test" INPUT_DEVICES="-wacom" VIDEO_CARDS="-nvidia"

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

  The above constraints are a subset of the following complete expression:
    devkit? ( screencast ) gtk-doc? ( introspection ) exactly-one-of ( elogind systemd ) test? ( screencast )

(dependency required by "gnome-base/gnome-shell-50.5::gentoo" [ebuild])
(dependency required by "gnome-extra/gnome-browser-connector-42.1-r1::gentoo" [ebuild])
(dependency required by "=gnome-extra/gnome-browser-connector-42.1-r1" [argument])