Starting from >=dev-util/capnproto-0.7.0 (from this commit https://github.com/capnproto/capnproto/commit/66730ef6c) it requires c++14 not just for itself but for every package that depends on it. For example =dev-util/rr-5.2.0 does not compile when =dev-util/capnproto-0.7.0 is installed but works fine with the previous =dev-util/capnproto-0.6.1 version. I'm not sure what's the correct way to deal with this, but for example media-sound/sonic-visualiser switched to c++14 because of this. (https://github.com/gentoo/gentoo/commit/0bbf3625a)