Port additional tests from #14579 (the issue is already fixed).

Lib/test/test_codecs.py

@@ -557,8 +557,19 @@ class UTF16LETest(ReadTest):
         )
 
     def test_errors(self):
-        self.assertRaises(UnicodeDecodeError, codecs.utf_16_le_decode,
-                          b"\xff", "strict", True)
+        tests = [
+            (b'\xff', '\ufffd'),
+            (b'A\x00Z', 'A\ufffd'),
+            (b'A\x00B\x00C\x00D\x00Z', 'ABCD\ufffd'),
+            (b'\x00\xd8', '\ufffd'),
+            (b'\x00\xd8A', '\ufffd'),
+            (b'\x00\xd8A\x00', '\ufffdA'),
+            (b'\x00\xdcA\x00', '\ufffdA'),
+        ]
+        for raw, expected in tests:
+            self.assertRaises(UnicodeDecodeError, codecs.utf_16_le_decode,
+                              raw, 'strict', True)
+            self.assertEqual(raw.decode('utf-16le', 'replace'), expected)
 
     def test_nonbmp(self):
         self.assertEqual("\U00010203".encode(self.encoding),
@@ -585,8 +596,19 @@ class UTF16BETest(ReadTest):
         )
 
     def test_errors(self):
-        self.assertRaises(UnicodeDecodeError, codecs.utf_16_be_decode,
-                          b"\xff", "strict", True)
+        tests = [
+            (b'\xff', '\ufffd'),
+            (b'\x00A\xff', 'A\ufffd'),
+            (b'\x00A\x00B\x00C\x00DZ', 'ABCD\ufffd'),
+            (b'\xd8\x00', '\ufffd'),
+            (b'\xd8\x00\xdc', '\ufffd'),
+            (b'\xd8\x00\x00A', '\ufffdA'),
+            (b'\xdc\x00\x00A', '\ufffdA'),
+        ]
+        for raw, expected in tests:
+            self.assertRaises(UnicodeDecodeError, codecs.utf_16_be_decode,
+                              raw, 'strict', True)
+            self.assertEqual(raw.decode('utf-16be', 'replace'), expected)
 
     def test_nonbmp(self):
         self.assertEqual("\U00010203".encode(self.encoding),