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diff --git a/t/t6424-merge-unrelated-index-changes.sh b/t/t6424-merge-unrelated-index-changes.sh
new file mode 100755
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+#!/bin/sh
+
+test_description="merges with unrelated index changes"
+
+. ./test-lib.sh
+
+# Testcase for some simple merges
+#   A
+#   o-------o B
+#    \
+#     \-----o C
+#      \
+#       \---o D
+#        \
+#         \-o E
+#          \
+#           o F
+#   Commit A: some file a
+#   Commit B: adds file b, modifies end of a
+#   Commit C: adds file c
+#   Commit D: adds file d, modifies beginning of a
+#   Commit E: renames a->subdir/a, adds subdir/e
+#   Commit F: empty commit
+
+test_expect_success 'setup trivial merges' '
+	test_seq 1 10 >a &&
+	git add a &&
+	test_tick && git commit -m A &&
+
+	git branch A &&
+	git branch B &&
+	git branch C &&
+	git branch D &&
+	git branch E &&
+	git branch F &&
+
+	git checkout B &&
+	echo b >b &&
+	echo 11 >>a &&
+	git add a b &&
+	test_tick && git commit -m B &&
+
+	git checkout C &&
+	echo c >c &&
+	git add c &&
+	test_tick && git commit -m C &&
+
+	git checkout D &&
+	test_seq 2 10 >a &&
+	echo d >d &&
+	git add a d &&
+	test_tick && git commit -m D &&
+
+	git checkout E &&
+	mkdir subdir &&
+	git mv a subdir/a &&
+	echo e >subdir/e &&
+	git add subdir &&
+	test_tick && git commit -m E &&
+
+	git checkout F &&
+	test_tick && git commit --allow-empty -m F
+'
+
+test_expect_success 'ff update' '
+	git reset --hard &&
+	git checkout A^0 &&
+
+	touch random_file && git add random_file &&
+
+	git merge E^0 &&
+
+	test_must_fail git rev-parse HEAD:random_file &&
+	test "$(git diff --name-only --cached E)" = "random_file" &&
+	test_path_is_file random_file &&
+	git rev-parse --verify :random_file
+'
+
+test_expect_success 'ff update, important file modified' '
+	git reset --hard &&
+	git checkout A^0 &&
+
+	mkdir subdir &&
+	touch subdir/e &&
+	git add subdir/e &&
+
+	test_must_fail git merge E^0 &&
+	test_path_is_file subdir/e &&
+	git rev-parse --verify :subdir/e &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'resolve, trivial' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge -s resolve C^0 &&
+	test_path_is_file random_file &&
+	git rev-parse --verify :random_file &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'resolve, non-trivial' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge -s resolve D^0 &&
+	test_path_is_file random_file &&
+	git rev-parse --verify :random_file &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'resolve, trivial, related file removed' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	git rm a &&
+	test_path_is_missing a &&
+
+	test_must_fail git merge -s resolve C^0 &&
+
+	test_path_is_missing a &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'resolve, non-trivial, related file removed' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	git rm a &&
+	test_path_is_missing a &&
+
+	# We also ask for recursive in order to turn off the "allow_trivial"
+	# setting in builtin/merge.c, and ensure that resolve really does
+	# correctly fail the merge (I guess this also tests that recursive
+	# correctly fails the merge, but the main thing we are attempting
+	# to test here is resolve and are just using the side effect of
+	# adding recursive to ensure that resolve is actually tested rather
+	# than the trivial merge codepath)
+	test_must_fail git merge -s resolve -s recursive D^0 &&
+
+	test_path_is_missing a &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'recursive' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge -s recursive C^0 &&
+	test_path_is_file random_file &&
+	git rev-parse --verify :random_file &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'recursive, when merge branch matches merge base' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge -s recursive F^0 &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'merge-recursive, when index==head but head!=HEAD' '
+	git reset --hard &&
+	git checkout C^0 &&
+
+	# Make index match B
+	git diff C B -- | git apply --cached &&
+	test_when_finished "git clean -fd" &&  # Do not leave untracked around
+	# Merge B & F, with B as "head"
+	git merge-recursive A -- B F > out &&
+	test_i18ngrep "Already up to date" out
+'
+
+test_expect_success 'recursive, when file has staged changes not matching HEAD nor what a merge would give' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	mkdir subdir &&
+	test_seq 1 10 >subdir/a &&
+	git add subdir/a &&
+	git rev-parse --verify :subdir/a >expect &&
+
+	# We have staged changes; merge should error out
+	test_must_fail git merge -s recursive E^0 2>err &&
+	git rev-parse --verify :subdir/a >actual &&
+	test_cmp expect actual &&
+	test_i18ngrep "changes to the following files would be overwritten" err
+'
+
+test_expect_success 'recursive, when file has staged changes matching what a merge would give' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	mkdir subdir &&
+	test_seq 1 11 >subdir/a &&
+	git add subdir/a &&
+	git rev-parse --verify :subdir/a >expect &&
+
+	# We have staged changes; merge should error out
+	test_must_fail git merge -s recursive E^0 2>err &&
+	git rev-parse --verify :subdir/a >actual &&
+	test_cmp expect actual &&
+	test_i18ngrep "changes to the following files would be overwritten" err
+'
+
+test_expect_success 'octopus, unrelated file touched' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge C^0 D^0 &&
+	test_path_is_missing .git/MERGE_HEAD &&
+	git rev-parse --verify :random_file &&
+	test_path_exists random_file
+'
+
+test_expect_success 'octopus, related file removed' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	git rm b &&
+
+	test_must_fail git merge C^0 D^0 &&
+	test_path_is_missing b &&
+	test_must_fail git rev-parse --verify :b &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'octopus, related file modified' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	echo 12 >>a && git add a &&
+	git rev-parse --verify :a >expect &&
+
+	test_must_fail git merge C^0 D^0 &&
+	test_path_is_file a &&
+	git rev-parse --verify :a >actual &&
+	test_cmp expect actual &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'ours' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge -s ours C^0 &&
+	test_path_is_file random_file &&
+	git rev-parse --verify :random_file &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'subtree' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge -s subtree E^0 &&
+	test_path_is_file random_file &&
+	git rev-parse --verify :random_file &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
+test_expect_success 'avoid failure due to stat-dirty files' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	# Make "a" be stat-dirty
+	test-tool chmtime =+1 a &&
+
+	# stat-dirty file should not prevent stash creation in builtin/merge.c
+	git merge -s resolve -s recursive D^0
+'
+
+test_expect_success 'with multiple strategies, recursive or ort failure do not early abort' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	test_seq 0 10 >a &&
+	git add a &&
+	git rev-parse :a >expect &&
+
+	sane_unset GIT_TEST_MERGE_ALGORITHM &&
+	test_must_fail git merge -s recursive -s ort -s octopus C^0 >output 2>&1 &&
+
+	grep "Trying merge strategy recursive..." output &&
+	grep "Trying merge strategy ort..." output &&
+	grep "Trying merge strategy octopus..." output &&
+	grep "No merge strategy handled the merge." output &&
+
+	# Changes to "a" should remain staged
+	git rev-parse :a >actual &&
+	test_cmp expect actual
+'
+
+test_done