This is the same thing as bug 563194, but with a different package.
dev-java/antlr-3.1.3-r3 is the only dev-java/antlr:3 ebuild (in my case pulled in by jython).
antlr-3.1.3-r3 depends on >=dev-java/antlr-2.7.7:0[java], but newer dev-java/antlr:0 ebuilds do not have that flag.
I noticed this because dev-java/stringtemplate-3.2.1 was just removed, and I cannot upgrade to stringtemplate-3.2.1-r1 because it depends on >=dev-java/antlr-2.7.7-r7:0, which conflicts with dev-java/antlr:3 because of this bug. I can downgrade to dev-java/stringtemplate-3.2 instead, but a fixed antlr:3 ebuild would be nice.
Sorry. As I mentioned in the other bug, this is a messy situation and it's hard to predict how things will interact. I have just committed dev-java/antlr-3.2:3, which depends on >=dev-java/antlr-2.7.7-r7:0 and should hopefully resolve the issue.
Thanks, confirmed that fixed the conflict.