After putting the line
into /etc/portage/package.keywords, emerge -a db wants to install (with ACCEPT_KEYWORDS=amd64) sys-libs/db-4.5.20_p2 which is keyworded ~amd64 and belongs to slot 4.5.
The problem happens also with ACCEPT_KEYWORDS=x86 and also with portage-184.108.40.206, see bug 181244 (comments #2 and #3).
Created attachment 121695 [details, diff]
add support for slot deps in package.keywords
Thanks, this is fixed in svn r6791.
This has been released in 2.1.3_rc1.