Install this version:
emerge -a =app-emacs/concurrent-9999# Copyright 1999-2021 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
NEED_EMACS="24.3"
inherit elisp git-r3
DESCRIPTION="Concurrent utility functions for Emacs Lisp"
HOMEPAGE="https://github.com/kiwanami/emacs-deferred/blob/master/README-concurrent.markdown"
EGIT_REPO_URI="https://github.com/kiwanami/emacs-deferred.git"
KEYWORDS="amd64 x86"
LICENSE="GPL-3"
SLOT="0"
SITEFILE="50${PN}-gentoo.el"
RDEPEND="
>=app-emacs/deferred-0.5.0
"