Created attachment 430284 [details] build log [ebuild N ] dev-ruby/json-1.8.2-r1 USE="doc {-test}" RUBY_TARGETS="ruby20 ruby21 -ruby19"
Created attachment 430286 [details] emerge --info
Can you confirm that this happens on a fresh install where ruby was not installed yet?
*** This bug has been marked as a duplicate of bug 572042 ***