funtoo ~ # emerge -auDN @world

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

Calculating dependencies... done!
[ebuild UD ] sys-fs/udev-init-scripts-27 [27-r1]
[blocks B ] <sys-apps/openrc-0.13 ("<sys-apps/openrc-0.13" is blocking sys-fs/udev-init-scripts-27)

• Error: The above package list contains packages which cannot be
• installed at the same time on the same system.

(sys-apps/openrc-0.12.4-r4:0/0::gentoo, installed) pulled in by
sys-apps/openrc required by (virtual/baselayout-0:0/0::gentoo, installed)
sys-apps/openrc required by (virtual/service-manager-0:0/0::gentoo, installed)
sys-apps/openrc required by @system
>=sys-apps/openrc-0.12.4-r2 required by (sys-apps/kmod-19:0/0::gentoo, installed)
sys-apps/openrc required by (sys-apps/baselayout-2.2.0-r6:0/0::gentoo, installed)

(sys-fs/udev-init-scripts-27:0/0::gentoo, ebuild scheduled for merge) pulled in by
>=sys-fs/udev-init-scripts-26 required by (sys-fs/eudev-3.1.2:0/0::gentoo, installed)

For more information about Blocked Packages, please refer to the following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):

https://wiki.gentoo.org/wiki/Handbook:X86/Working/Portage#Blocked_packages