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

Calculating dependencies ... done!
Dependency resolution took 0.91 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 media-gfx/simple-scan-49.1::gentoo
# required by =media-gfx/simple-scan-49.1 (argument)
>=gui-libs/libadwaita-1.9.4 vala

!!! The ebuild selected to satisfy "gui-libs/libadwaita:1[vala]" has unmet requirements.
- gui-libs/libadwaita-1.9.4::gentoo USE="-examples -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:
    gtk-doc? ( introspection ) vala? ( introspection )

(dependency required by "media-gfx/simple-scan-49.1::gentoo" [ebuild])
(dependency required by "=media-gfx/simple-scan-49.1" [argument])