[PATCH] make.defaults: prevent USE="${USE} ..." misbehavior

Discard parent profile USE from the expand_map variable before
it is used to evaluate a child profile. This prevents accidents
triggered by USE="${USE} ..." settngs at the top of make.defaults
which caused parent profile USE to override parent profile
package.use settings.

Bug: https://bugs.gentoo.org/771549
Signed-off-by: Zac Medico <[email protected]>
---
 lib/portage/package/ebuild/config.py | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/lib/portage/package/ebuild/config.py b/lib/portage/package/ebuild/config.py
index e5ec681af..638c72959 100644
--- a/lib/portage/package/ebuild/config.py
+++ b/lib/portage/package/ebuild/config.py
@@ -612,9 +612,16 @@ class config:
 
 			mygcfg = {}
 			if profiles_complex:
-				mygcfg_dlists = [getconfig(os.path.join(x.location, "make.defaults"),
-					tolerant=tolerant, expand=expand_map, recursive=x.portage1_directories)
-					for x in profiles_complex]
+				mygcfg_dlists = []
+				for x in profiles_complex:
+					# Prevent accidents triggered by USE="${USE} ..." settings
+					# at the top of make.defaults which caused parent profile
+					# USE to override parent profile package.use settings.
+					expand_map.pop("USE", None)
+					mygcfg_dlists.append(
+						getconfig(os.path.join(x.location, "make.defaults"),
+						tolerant=tolerant, expand=expand_map,
+						recursive=x.portage1_directories))
 				self._make_defaults = mygcfg_dlists
 				mygcfg = stack_dicts(mygcfg_dlists,
 					incrementals=self.incrementals)
-- 
2.26.2