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

Calculating dependencies ... done!
Dependency resolution took 0.98 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 mail-client/geary-46.0-r1::gentoo
# required by =mail-client/geary-46.0-r1 (argument)
>=x11-libs/gtk+-3.24.52 introspection
# required by mail-client/geary-46.0-r1::gentoo
# required by =mail-client/geary-46.0-r1 (argument)
>=net-libs/webkit-gtk-2.54.1-r400:4.1 introspection
# required by mail-client/geary-46.0-r1::gentoo
# required by =mail-client/geary-46.0-r1 (argument)
>=dev-libs/gmime-3.2.15-r2 vala
# required by mail-client/geary-46.0-r1::gentoo
# required by =mail-client/geary-46.0-r1 (argument)
>=app-crypt/gcr-3.41.2-r2:0 introspection vala
# required by mail-client/geary-46.0-r1::gentoo
# required by =mail-client/geary-46.0-r1 (argument)
>=dev-libs/libgee-0.20.8 introspection
# required by mail-client/geary-46.0-r1::gentoo
# required by =mail-client/geary-46.0-r1 (argument)
>=media-libs/gsound-1.0.3 vala

!!! The ebuild selected to satisfy "media-libs/gsound[vala]" has unmet requirements.
- media-libs/gsound-1.0.3::gentoo USE="-gtk-doc -introspection -vala"

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

(dependency required by "mail-client/geary-46.0-r1::gentoo" [ebuild])
(dependency required by "=mail-client/geary-46.0-r1" [argument])