test(unittest.util): Add tests for sorted_list_difference tail deduplication

Exercise the except-IndexError path where one list is exhausted
and the remaining tail of the other list contains duplicates.
These cases were previously untested and would have passed even
without the fix in the preceding commit.

Author: stefanzetzsche

Lib/test/test_unittest/test_util.py

@@ -26,6 +26,17 @@ def test_sorted_list_difference(self):
         self.assertEqual(sorted_list_difference([2], [1, 1]), ([2], [1]))
         self.assertEqual(sorted_list_difference([1, 2], [1, 1]), ([2], []))
 
+    def test_sorted_list_difference_tail_deduplication(self):
+        # Tail deduplication when one list is exhausted before the other.
+        # These exercise the except-IndexError path in sorted_list_difference.
+        self.assertEqual(sorted_list_difference([], [0, 0]), ([], [0]))
+        self.assertEqual(sorted_list_difference([0, 0], []), ([0], []))
+        self.assertEqual(sorted_list_difference([], [1, 1, 2, 2]), ([], [1, 2]))
+        self.assertEqual(sorted_list_difference([1, 1, 2, 2], []), ([1, 2], []))
+        # One list exhausts mid-way, leaving duplicated tail in the other.
+        self.assertEqual(sorted_list_difference([1], [1, 2, 2, 3, 3]), ([], [2, 3]))
+        self.assertEqual(sorted_list_difference([1, 2, 2, 3, 3], [1]), ([2, 3], []))
+
     def test_unorderable_list_difference(self):
         self.assertEqual(unorderable_list_difference([], []), ([], []))
         self.assertEqual(unorderable_list_difference([1, 2], []), ([2, 1], []))