With USE=doc, gnome-desktop-3.0.0 and 2.32.1 collide due to trying to install their license documentation files in the same location.
USE="doc" emerge gnome-desktop:3
>>> Installing (6 of 13) gnome-base/gnome-desktop-3.0.0
* checking 46 files for package collisions
* This package will overwrite one or more files that may belong to other
* packages (see list below). You can use a command such as `portageq
* owners / <filename>` to identify the installed package that owns a
* file. If portageq reports that only one package owns a file then do
* NOT file a bug report. A bug report is only useful if it identifies at
* least two or more packages that are known to install the same file(s).
* If a collision occurs and you can not explain where the file came from
* then you should simply ignore the collision since there is not enough
* information to determine if a real problem exists. Please do NOT file
* a bug report at http://bugs.gentoo.org unless you report exactly which
* two packages install the same file(s). Once again, please do NOT file
* a bug report unless you have completely understood the above message.
* Detected file collision(s):
* Searching all installed packages for file collisions...
* Press Ctrl-C to Stop
* Package 'gnome-base/gnome-desktop-3.0.0' NOT merged due to file
* collisions. If necessary, refer to your elog messages for the whole
* content of the above message.
Created attachment 268813 [details, diff]
adds blocking dependency
It is ok. USE="-doc" resolves the conflict between vers 2 and 3 (since 2.32.1 and 3.0.0)
But there was no block in ebuild. Here is a simple patch to add blocking
This was fixed quite some time ago.