media-sound/jack-audio-connection-kit-0.103.0 is present in the official portage tree, and in the pro-audio overlay. It has KEYWORDS="x86" in the official tree, and KEYWORDS="~x86" in the pro-audio overlay.
When I try to (re-)install it, emerge fails:
$ sudo emerge -1 =media-sound/jack-audio-connection-kit-0.103.0
These are the packages that would be merged, in order:
Calculating dependencies /
!!! All ebuilds that could satisfy "=media-sound/jack-audio-connection-kit-0.103.0" have been masked.
!!! One of the following masked packages is required to complete your request:
- media-sound/jack-audio-connection-kit-0.103.0 (masked by: ~x86 keyword)
If multiple ebuilds for an identical version of a package are available (from the official tree and overlays, or from multiple overlays), at least one of the versions is marked stable, and no version is unmasked via /etc/portage/package.keywords (for example), emerge should choose one among all stable/unmasked versions instead of preferring only the overlay versions (which may be masked/not stable).
This *is* why it's called an overlay. It.. lays itself over the official tree.
Ummm… IIRC, if there is a newer version in the official tree and not in the overlay, the newer version is used. IMHO, this should also happen in the case I described in comment 0.
So if it lays itself over the official tree, that tree should “shine through” for ebuilds where the overlay cannot be used (due to masks etc.); I’d say the overlay should become “transparent” in these areas.
*** This bug has been marked as a duplicate of bug 185377 ***