gh-119180: Disallow instantiation of ConstEvaluator objects (#124561)

Author: JelleZijlstra

Lib/test/test_type_params.py

@@ -1452,3 +1452,14 @@ def f[T: (int, str)](): pass
                 self.assertEqual(annotationlib.call_evaluate_function(case.evaluate_constraints, annotationlib.Format.VALUE), (int, str))
                 self.assertEqual(annotationlib.call_evaluate_function(case.evaluate_constraints, annotationlib.Format.FORWARDREF), (int, str))
                 self.assertEqual(annotationlib.call_evaluate_function(case.evaluate_constraints, annotationlib.Format.SOURCE), '(int, str)')
+
+    def test_const_evaluator(self):
+        T = TypeVar("T", bound=int)
+        self.assertEqual(repr(T.evaluate_bound), "<constevaluator <class 'int'>>")
+
+        ConstEvaluator = type(T.evaluate_bound)
+
+        with self.assertRaisesRegex(TypeError, r"cannot create '_typing\._ConstEvaluator' instances"):
+            ConstEvaluator()  # This used to segfault.
+        with self.assertRaisesRegex(TypeError, r"cannot set 'attribute' attribute of immutable type '_typing\._ConstEvaluator'"):
+            ConstEvaluator.attribute = 1

Objects/typevarobject.c

@@ -151,7 +151,7 @@ constevaluator_clear(PyObject *self)
 }
 
 static PyObject *
-constevaluator_repr(PyObject *self, PyObject *repr)
+constevaluator_repr(PyObject *self)
 {
     PyObject *value = ((constevaluatorobject *)self)->value;
     return PyUnicode_FromFormat("<constevaluator %R>", value);
@@ -242,7 +242,8 @@ static PyType_Slot constevaluator_slots[] = {
 PyType_Spec constevaluator_spec = {
     .name = "_typing._ConstEvaluator",
     .basicsize = sizeof(constevaluatorobject),
-    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_IMMUTABLETYPE,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_IMMUTABLETYPE
+        | Py_TPFLAGS_DISALLOW_INSTANTIATION,
     .slots = constevaluator_slots,
 };