From: Junio C Hamano <gitster@pobox.com>
Date: Fri, 25 Sep 2026 04:12:49 GMT
Subject: Re: [PATCH v2 4/4] builtin/stash: merge index in-core
Message-ID: <xmqqpky2x932.fsf@gitster.g>
In-Reply-To: <xmqqse2yz4y4.fsf@gitster.g>

Junio C Hamano <gitster@pobox.com> writes:

> The new test in the attached patch will fail with this step but if
> we revert the changes to builtin/stash.c in this step, it passes.

Oh, and with the change to the code, it passes again.

diff --git i/builtin/stash.c w/builtin/stash.c
index 219ca457be..44d962cc5d 100644
--- i/builtin/stash.c
+++ w/builtin/stash.c
@@ -639,7 +639,7 @@ static enum stash_apply_result do_apply_stash(const char *prefix,
 			merge = lookup_tree(o.repo, &info->i_tree);
 			merge_base = lookup_tree(o.repo, &info->b_tree);
 
-			merge_incore_nonrecursive(&o, head, merge, merge_base,
+			merge_incore_nonrecursive(&o, merge_base, merge, head,
 						  &result);
 
 			if (!result.clean)
diff --git i/t/t3903-stash.sh w/t/t3903-stash.sh
index 3958ab3c8d..0a87e62b11 100755
--- i/t/t3903-stash.sh
+++ w/t/t3903-stash.sh
@@ -374,6 +374,38 @@ test_expect_success 'stash apply -q --index refreshes the index' '
 	test_cmp expect actual
 '
 
+
+test_expect_success 'stash apply --index does not revert unrelated upstream index changes' '
+	test_when_finished "rm -fr playpen" &&
+	mkdir playpen &&
+	(
+		cd playpen &&
+		git init &&
+		echo "base1" >file1 &&
+		echo "base2" >file2 &&
+		git add file1 file2 &&
+		git commit -m "initial base" &&
+
+		# Make a staged change to file1 and stash it
+		echo "staged1" >file1 &&
+		git add file1 &&
+		git stash &&
+
+		# Upstream advances by modifying unrelated file2
+		echo "upstream2" >file2 &&
+		git add file2 &&
+		git commit -m "upstream change to file2" &&
+
+		# Apply the stash with --index
+		git stash apply --index &&
+
+		# Verify working tree and index state
+		test "$(git show :file1)" = "staged1" &&
+		test "$(git show :file2)" = "upstream2" &&
+		test "$(git show HEAD:file2)" = "upstream2"
+	)
+'
+
 test_expect_success 'stash apply --index leaves everything untouched on failure' '
 	git reset --hard &&
 	echo test >other-file &&