Doing a weekly '#emerge -auDN @world' and sys-libs/readline was one of the many packages to be updated.
If failed to emerge with:

>>>  '/var/tmp/portage/sys-libs/readline-8.2/temp/build.log'

 * Messages for package sys-libs/readline-8.2:

 * ERROR: sys-libs/readline-8.2::core-kit failed (prepare phase):
 **   patch -p1  failed with /var/tmp/portage/sys-libs/readline-8.2/files/readline-8.0-darwin-shlib-versioning.patch*
 *
 * Call stack:
 *               ebuild.sh, line   93:  Called src_prepare
 *             environment, line 2090:  Called default
 *      phase-functions.sh, line  855:  Called default_src_prepare
 *      phase-functions.sh, line  920:  Called __eapi6_src_prepare
 *             environment, line  195:  Called eapply '/var/tmp/portage/sys-libs/readline-8.2/files/readline-5.0-no_rpath.patch' '/var/tmp/portage/sys-libs/readline-8.2/files/readline-6.2-rlfe-tgoto.patch' '/var/tmp/portage/sys-libs/readline-8.2/files/readline-7.0-headers.patch' '/var/tmp/portage/sys-libs/readline-8.2/files/readline-8.0-headers.patch' '/var/tmp/portage/sys-libs/readline-8.2/files/readline-8.0-darwin-shlib-versioning.patch' '/var/tmp/portage/sys-libs/readline-8.2/files/readline-8.1-windows-signals.patch'
 *             environment, line  604:  Called _eapply_patch '/var/tmp/portage/sys-libs/readline-8.2/files/readline-8.0-darwin-shlib-versioning.patch'
 *             environment, line  542:  Called __helpers_die 'patch -p1  failed with /var/tmp/portage/sys-libs/readline-8.2/files/readline-8.0-darwin-shlib-versioning.patch'
 *   isolated-functions.sh, line  112:  Called die
 * The specific snippet of code:
 *              die "$@"