gh-83731: Test that isinstance() ignores tuple subclass methods (#158000)

Add a regression test to make sure isinstance() reads the items of a
tuple subclass directly, without calling its __iter__(), __getitem__()
or __len__().

(cherry picked from commit 21a36aa)

Author: vasiliyk

Lib/test/test_isinstance.py

@@ -263,6 +263,20 @@ def test_subclass_tuple(self):
         self.assertEqual(True, issubclass(int, (int, (float, int))))
         self.assertEqual(True, issubclass(str, (str, (Child, str))))
 
+    def test_tuple_subclass_methods_not_called(self):
+        # gh-83731: make sure isinstance() reads the items of a tuple
+        # subclass directly, without calling its __iter__(),
+        # __getitem__() or __len__().
+        class T(tuple):
+            def __iter__(self):
+                raise AssertionError('__iter__ called')
+            def __getitem__(self, index):
+                raise AssertionError('__getitem__ called')
+            def __len__(self):
+                raise AssertionError('__len__ called')
+
+        self.assertTrue(isinstance(1, T((int,))))
+
     def test_subclass_recursion_limit(self):
         # make sure that issubclass raises RecursionError before the C stack is
         # blown