Add tests for issue #18183.

Lib/test/test_unicode.py

@@ -566,9 +566,6 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertEqual('\U0008fffe'.lower(), '\U0008fffe')
         self.assertEqual('\u2177'.lower(), '\u2177')
 
-        # See issue #18183 for this one.
-        '\U00010000\U00100000'.lower()
-
     def test_casefold(self):
         self.assertEqual('hello'.casefold(), 'hello')
         self.assertEqual('hELlo'.casefold(), 'hello')
@@ -686,6 +683,17 @@ class UnicodeTest(string_tests.CommonTest,
 
         self.assertRaises(TypeError, "abc".__contains__)
 
+    def test_issue18183(self):
+        '\U00010000\U00100000'.lower()
+        '\U00010000\U00100000'.casefold()
+        '\U00010000\U00100000'.upper()
+        '\U00010000\U00100000'.capitalize()
+        '\U00010000\U00100000'.title()
+        '\U00010000\U00100000'.swapcase()
+        '\U00100000'.center(3, '\U00010000')
+        '\U00100000'.ljust(3, '\U00010000')
+        '\U00100000'.rjust(3, '\U00010000')
+
     def test_format(self):
         self.assertEqual(''.format(), '')
         self.assertEqual('a'.format(), 'a')
@@ -2016,6 +2024,10 @@ class UnicodeTest(string_tests.CommonTest,
         # test "%c"
         self.assertEqual(PyUnicode_FromFormat(b'%c', c_int(0xabcd)), '\uabcd')
         self.assertEqual(PyUnicode_FromFormat(b'%c', c_int(0x10ffff)), '\U0010ffff')
+        # Issue #18183
+        self.assertEqual(
+            PyUnicode_FromFormat(b'%c%c', c_int(0x10000), c_int(0x100000)),
+            '\U00010000\U00100000')
 
         # test "%"
         self.assertEqual(PyUnicode_FromFormat(b'%'), '%')