Install this version:
emerge -a =app-emacs/dash-9999
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =app-emacs/dash-9999
Or alternatively:
emerge --autounmask-write -a =app-emacs/dash-9999
# Copyright 1999-2021 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
inherit elisp git-r3
DESCRIPTION="A modern list library for Emacs"
HOMEPAGE="https://github.com/magnars/dash.el"
EGIT_REPO_URI="https://github.com/magnars/dash.el.git"
EGIT_BRANCH="master"
LICENSE="GPL-3+"
SLOT="0"
KEYWORDS="~amd64 ~arm ~ppc64 ~x86 ~amd64-linux ~x86-linux"
BDEPEND="sys-apps/texinfo"
SITEFILE="50${PN}-gentoo.el"
ELISP_TEXINFO="dash.texi"
DOCS="README.md"