ebuild.sh: disable patsub_replacement in Bash 5.2

patsub_replacement is a new option in bash-5.2 that is also default-on
in that release. The default value is not gated by BASH_COMPAT (see bug #881383),
hence we need to disable it for older Bashes to avoid behaviour changes in ebuilds
and eclasses.

Thanks to Kerin for both raising this & being persistent with trying
to get Bash 5.2 to be suitable for use in Gentoo.

Bug: https://bugs.gentoo.org/881383
Thanks-to: Kerin Millar <kfm@plushkava.net>
Signed-off-by: Sam James <sam@gentoo.org>

NEWS

@@ -17,6 +17,11 @@ Features:
   scratch using a crossdev environment under /usr/${CHOST}.
 
 Bug fixes:
+* ebuild: Handle Bash 5.2's change in behavior which enables the shopt
+  'patsub_replacement' by default. This is needed to avoid breaking existing
+  working ebuilds. Future EAPIs will need to adjust the logic
+  added by this change. See bug #881383.
+
 * sync: Clobber repositories using sync-type=git to match rsync behavior. This
   helps with issues where git-synced repositories can become confused
   if the remote is a CDN and then starts to diverge, preventing further

bin/ebuild.sh

@@ -18,6 +18,7 @@ source "${PORTAGE_BIN_PATH}/isolated-functions.sh" || exit 1
 # used instead.
 __check_bash_version() {
 	# Figure out which min version of bash we require.
+	# Adjust patsub_replacement logic below on new EAPI!
 	local maj min
 	if ___eapi_bash_3_2 ; then
 		maj=3 min=2
@@ -50,6 +51,19 @@ __check_bash_version() {
 	if ___eapi_bash_3_2 && [[ ${BASH_VERSINFO[0]} -gt 3 ]] ; then
 		shopt -s compat32
 	fi
+
+	# patsub_replacement is a new option in bash-5.2 that is also default-on
+	# in that release. The default value is not gated by BASH_COMPAT (see bug #881383),
+	# hence we need to disable it for older Bashes to avoid behaviour changes in ebuilds
+	# and eclasses.
+	#
+	# New EAPI note: a newer EAPI (after 8) may well adopt Bash 5.2 as its minimum version.
+	# If it does, this logic will need to be adjusted to only disable patsub_replacement
+	# for < ${new_api}!
+	if (( BASH_VERSINFO[0] >= 6 || ( BASH_VERSINFO[0] == 5 && BASH_VERSINFO[1] >= 2 ) )) ; then
+		shopt -u patsub_replacement
+	fi
+
 }
 __check_bash_version