Recent versions of portage have brought some confusion with overlays and overriding eclasses. Due to QA issues, zmedico has decided to disable automatic eclass overrides from overlays.

This means that if, for example, you’re using the “kde-testing” overlay, you’ll have to manually tell portage to use any updated eclasses available in the overlay. If you don’t, you might end up with broken packages, depending on the overlay. You should check out the overlay’s guides and/or chat with the developers to find out if overriding is necessary/optional.

To tell portage to use the kde-testing overlay’s eclasses [if any], create the file:
/etc/portage/repos.conf

with the following content:

[gentoo]
eclass-overrides = kde

We’re using “kde” instead of “kde-testing”, because that’s the actual overlay name (found in OVERLAY_FOLDER/profiles/repo_name)

For more information on this issue, read gentoo bug #265747