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

Calculating dependencies ... done!
Dependency resolution took 0.93 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-extra/gnome-clocks-50.0::gentoo
# required by =gnome-extra/gnome-clocks-50.0 (argument)
>=dev-libs/libgweather-4.6.0 vala

!!! The ebuild selected to satisfy ">=dev-libs/libgweather-4.2.0:4[vala]" has unmet requirements.
- dev-libs/libgweather-4.6.0::gentoo USE="-gtk-doc -introspection -test -vala"

  The following REQUIRED_USE flag constraints are unsatisfied:
    vala? ( introspection )

  The above constraints are a subset of the following complete expression:
    vala? ( introspection ) gtk-doc? ( introspection )

(dependency required by "gnome-extra/gnome-clocks-50.0::gentoo" [ebuild])
(dependency required by "=gnome-extra/gnome-clocks-50.0" [argument])