2 files changed, 119 insertions, 0 deletions
diff --git a/merge-recursive.c b/merge-recursive.c
index a0c804c817..2da4333439 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -447,6 +447,30 @@ static void flush_buffer(int fd, const char *buf, unsigned long size)
 	}
 }
 
+static int would_lose_untracked(const char *path)
+{
+	int pos = cache_name_pos(path, strlen(path));
+
+	if (pos < 0)
+		pos = -1 - pos;
+	while (pos < active_nr &&
+	       !strcmp(path, active_cache[pos]->name)) {
+		/*
+		 * If stage #0, it is definitely tracked.
+		 * If it has stage #2 then it was tracked
+		 * before this merge started.  All other
+		 * cases the path was not tracked.
+		 */
+		switch (ce_stage(active_cache[pos])) {
+		case 0:
+		case 2:
+			return 0;
+		}
+		pos++;
+	}
+	return file_exists(path);
+}
+
 static int make_room_for_path(const char *path)
 {
 	int status;
@@ -462,6 +486,14 @@ static int make_room_for_path(const char *path)
 		die(msg, path, "");
 	}
 
+	/*
+	 * Do not unlink a file in the work tree if we are not
+	 * tracking it.
+	 */
+	if (would_lose_untracked(path))
+		return error("refusing to lose untracked file at '%s'",
+			     path);
+
 	/* Successful unlink is good.. */
 	if (!unlink(path))
 		return 0;
diff --git a/t/t7607-merge-overwrite.sh b/t/t7607-merge-overwrite.sh
new file mode 100755
index 0000000000..49f4e1599a
--- /dev/null
+++ b/t/t7607-merge-overwrite.sh
@@ -0,0 +1,87 @@
+#!/bin/sh
+
+test_description='git-merge
+
+Do not overwrite changes.'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	echo c0 > c0.c &&
+	git add c0.c &&
+	git commit -m c0 &&
+	git tag c0 &&
+	echo c1 > c1.c &&
+	git add c1.c &&
+	git commit -m c1 &&
+	git tag c1 &&
+	git reset --hard c0 &&
+	echo c2 > c2.c &&
+	git add c2.c &&
+	git commit -m c2 &&
+	git tag c2 &&
+	git reset --hard c1 &&
+	echo "c1 a" > c1.c &&
+	git add c1.c &&
+	git commit -m "c1 a" &&
+	git tag c1a &&
+	echo "VERY IMPORTANT CHANGES" > important
+'
+
+test_expect_success 'will not overwrite untracked file' '
+	git reset --hard c1 &&
+	cat important > c2.c &&
+	! git merge c2 &&
+	test_cmp important c2.c
+'
+
+test_expect_success 'will not overwrite new file' '
+	git reset --hard c1 &&
+	cat important > c2.c &&
+	git add c2.c &&
+	! git merge c2 &&
+	test_cmp important c2.c
+'
+
+test_expect_success 'will not overwrite staged changes' '
+	git reset --hard c1 &&
+	cat important > c2.c &&
+	git add c2.c &&
+	rm c2.c &&
+	! git merge c2 &&
+	git checkout c2.c &&
+	test_cmp important c2.c
+'
+
+test_expect_success 'will not overwrite removed file' '
+	git reset --hard c1 &&
+	git rm c1.c &&
+	git commit -m "rm c1.c" &&
+	cat important > c1.c &&
+	! git merge c1a &&
+	test_cmp important c1.c
+'
+
+test_expect_success 'will not overwrite re-added file' '
+	git reset --hard c1 &&
+	git rm c1.c &&
+	git commit -m "rm c1.c" &&
+	cat important > c1.c &&
+	git add c1.c &&
+	! git merge c1a &&
+	test_cmp important c1.c
+'
+
+test_expect_success 'will not overwrite removed file with staged changes' '
+	git reset --hard c1 &&
+	git rm c1.c &&
+	git commit -m "rm c1.c" &&
+	cat important > c1.c &&
+	git add c1.c &&
+	rm c1.c &&
+	! git merge c1a &&
+	git checkout c1.c &&
+	test_cmp important c1.c
+'
+
+test_done