Subclasses of string can no longer be interned. The semantics of

interning were not clear here -- a subclass could be mutable, for
example -- and had bugs.  Explicitly interning a subclass of string
via intern() will raise a TypeError.  Internal operations that attempt
to intern a string subclass will have no effect.

Added a few tests to test_builtin that includes the old buggy code and
verifies that calls like PyObject_SetAttr() don't fail.  Perhaps these
tests should have gone in test_string.

Python/bltinmodule.c

@@ -1035,6 +1035,11 @@ builtin_intern(PyObject *self, PyObject *args)
 	PyObject *s;
 	if (!PyArg_ParseTuple(args, "S:intern", &s))
 		return NULL;
+	if (!PyString_CheckExact(s)) {
+		PyErr_SetString(PyExc_TypeError,
+				"can't intern subclass of string");
+		return NULL;
+	}
 	Py_INCREF(s);
 	PyString_InternInPlace(&s);
 	return s;