atlas ~ # emerge -uDNvp @world

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

Calculating dependencies... done!
[ebuild  N    #] gnome-base/gnome-keyring-3.20.0::gentoo  USE="caps filecaps pam ssh-agent (-selinux) {-test}" 0 KiB
[ebuild  rR    ] sys-libs/ncurses-5.9-r5:0/5::gentoo  USE="cxx gpm unicode -ada -debug -doc -minimal -profile -static-libs -tinfo -trace" ABI_X86="32 (64) (-x32)" 0 KiB
[ebuild  r  U #] sys-libs/ncurses-6.0-r1:0/6::gentoo [5.9-r5:0/5::gentoo] USE="cxx gpm unicode -ada -debug -doc -minimal -profile -static-libs {-test%} -threads% -tinfo -trace" ABI_X86="32 (64) (-x32)" 3,059 KiB
[ebuild     U  ] dev-util/cmake-3.6.3::gentoo [3.5.2-r1::gentoo] USE="ncurses -doc -emacs -qt5 -system-jsoncpp {-test}" 6,758 KiB
[ebuild     U  ] www-client/chromium-55.0.2883.75::gentoo [54.0.2840.100::gentoo] USE="cups (gn) hangouts (pic) proprietary-codecs pulseaudio suid system-ffmpeg tcmalloc widevine -custom-cflags -gnome -gnome-keyring (-gtk3) -kerberos (-neon) (-selinux) {-test}" L10N="am ar bg bn ca cs da de el en-GB es es-419 et fa fi fil fr gu he hi hr hu id it ja kn ko lt lv ml mr ms nb nl pl pt-BR pt-PT ro ru sk sl sr sv sw ta te th tr uk vi zh-CN zh-TW" 0 KiB
[ebuild  rR    ] sys-devel/llvm-3.7.1-r3:0/3.7.1::gentoo  USE="clang libffi ncurses python static-analyzer xml -debug -doc -gold -libedit -lldb -multitarget -ocaml {-test}" ABI_X86="32 (64) (-x32)" PYTHON_TARGETS="python2_7" VIDEO_CARDS="-radeon" 0 KiB

Total: 6 packages (3 upgrades, 1 new, 2 reinstalls), Size of downloads: 9,817 KiB

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

sys-libs/ncurses:0

  (sys-libs/ncurses-6.0-r1:0/6::gentoo, ebuild scheduled for merge) pulled in by
    (no parents that aren't satisfied by other packages in this slot)

  (sys-libs/ncurses-5.9-r5:0/5::gentoo, ebuild scheduled for merge) pulled in by
    sys-libs/ncurses:0/5= required by (net-irc/irssi-0.8.20-r1:0/0::gentoo, installed)
                    ^^^^^
    (and 44 more with the same problem)

NOTE: Use the '--verbose-conflicts' option to display parents omitted above

It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously.  If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously. You may want to try a larger value of
the --backtrack option, such as --backtrack=30, in order to see if
that will solve this conflict automatically.

For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.


The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by sys-devel/llvm-3.7.1-r3::gentoo[ncurses]
# required by @selected
# required by @world (argument)
=sys-libs/ncurses-5.9-r5 abi_x86_32
# required by media-libs/mesa-12.0.1::gentoo[llvm]
# required by x11-base/xorg-server-1.18.4::gentoo[-minimal]
# required by @selected
# required by @world (argument)
>=sys-devel/llvm-3.7.1-r3 abi_x86_32

!!! All ebuilds that could satisfy ">=sys-devel/llvm-3.6.0:=[abi_x86_32(-)?,abi_x86_64(-)?,abi_x86_x32(-)?,abi_mips_n32(-)?,abi_mips_n64(-)?,abi_mips_o32(-)?,abi_ppc_32(-)?,abi_ppc_64(-)?,abi_s390_32(-)?,abi_s390_64(-)?]" have been masked.
!!! One of the following masked packages is required to complete your request:
- sys-devel/llvm-9999::gentoo (masked by: missing keyword)
- sys-devel/llvm-3.9.0-r1::gentoo (masked by: ~amd64 keyword)
- sys-devel/llvm-3.8.1-r2::gentoo (masked by: ~amd64 keyword)
- sys-devel/llvm-3.8.1::gentoo (masked by: ~amd64 keyword)
- sys-devel/llvm-3.7.1-r3::gentoo (masked by: )
- sys-devel/llvm-3.6.2::gentoo (masked by: ~amd64 keyword)

(dependency required by "media-libs/mesa-12.0.1::gentoo" [installed])
(dependency required by "x11-base/xorg-server-1.18.4::gentoo[-minimal]" [installed])
(dependency required by "@selected" [set])
(dependency required by "@world" [argument])
For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.