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

Calculating dependencies ... done!
Dependency resolution took 1.23 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-sound/mixxx-2.5.4::gentoo
# required by =media-sound/mixxx-2.5.4 (argument)
>=dev-qt/qtbase-6.11.2-r2 icu
# required by media-sound/mixxx-2.5.4::gentoo
# required by =media-sound/mixxx-2.5.4 (argument)
>=dev-qt/qt5compat-6.11.2 icu

 * In order to avoid wasting time, backtracking has terminated early
 * due to the above autounmask change(s). The --autounmask-backtrack=y
 * option can be used to force further backtracking, but there is no
 * guarantee that it will produce a solution.