[3.13] pythongh-78465: Fix error message for cls.__new__(cls, ...) where cls is not instantiable (pythonGH-135981)

Previous error message suggested to use cls.__new__(), which
obviously does not work. Now the error message is the same as for
cls(...).
(cherry picked from commit c45f4f3)

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: serhiy-storchaka

Lib/test/support/__init__.py

@@ -2269,6 +2269,7 @@ def check_disallow_instantiation(testcase, tp, *args, **kwds):
         qualname = f"{name}"
     msg = f"cannot create '{re.escape(qualname)}' instances"
     testcase.assertRaisesRegex(TypeError, msg, tp, *args, **kwds)
+    testcase.assertRaisesRegex(TypeError, msg, tp.__new__, tp, *args, **kwds)
 
 def get_recursion_depth():
     """Get the recursion depth of the caller function.

Lib/test/test_sys.py

@@ -894,12 +894,7 @@ def test_sys_flags(self):
     def assert_raise_on_new_sys_type(self, sys_attr):
         # Users are intentionally prevented from creating new instances of
         # sys.flags, sys.version_info, and sys.getwindowsversion.
-        arg = sys_attr
-        attr_type = type(sys_attr)
-        with self.assertRaises(TypeError):
-            attr_type(arg)
-        with self.assertRaises(TypeError):
-            attr_type.__new__(attr_type, arg)
+        support.check_disallow_instantiation(self, type(sys_attr), sys_attr)
 
     def test_sys_flags_no_instantiation(self):
         self.assert_raise_on_new_sys_type(sys.flags)

Lib/test/test_types.py

@@ -3,7 +3,7 @@
 from test.support import (
     run_with_locale, is_apple_mobile, cpython_only,
     iter_builtin_types, iter_slot_wrappers,
-    MISSING_C_DOCSTRINGS,
+    MISSING_C_DOCSTRINGS, check_disallow_instantiation,
 )
 from test.test_import import no_rerun
 from test.support.script_helper import assert_python_ok
@@ -1101,6 +1101,22 @@ def test_or_type_operator_reference_cycle(self):
         self.assertLessEqual(sys.gettotalrefcount() - before, leeway,
                              msg='Check for union reference leak.')
 
+<<<<<<< HEAD
+=======
+    def test_instantiation(self):
+        check_disallow_instantiation(self, types.UnionType)
+        self.assertIs(int, types.UnionType[int])
+        self.assertIs(int, types.UnionType[int, int])
+        self.assertEqual(int | str, types.UnionType[int, str])
+
+        for obj in (
+            int | typing.ForwardRef("str"),
+            typing.Union[int, "str"],
+        ):
+            self.assertIsInstance(obj, types.UnionType)
+            self.assertEqual(obj.__args__, (int, EqualToForwardRef("str")))
+
+>>>>>>> c45f4f3ebe3 (gh-78465: Fix error message for cls.__new__(cls, ...) where cls is not instantiable (GH-135981))
 
 class MappingProxyTests(unittest.TestCase):
     mappingproxy = types.MappingProxyType

Misc/NEWS.d/next/Core and Builtins/2025-06-26-15-25-51.gh-issue-78465.MbDN8X.rst

@@ -0,0 +1,2 @@
+Fix error message for ``cls.__new__(cls, ...)`` where ``cls`` is not
+instantiable builtin or extension type (with ``tp_new`` set to ``NULL``).

Objects/typeobject.c

@@ -9087,6 +9087,11 @@ tp_new_wrapper(PyObject *self, PyObject *args, PyObject *kwds)
     /* If staticbase is NULL now, it is a really weird type.
        In the spirit of backwards compatibility (?), just shut up. */
     if (staticbase && staticbase->tp_new != type->tp_new) {
+        if (staticbase->tp_new == NULL) {
+            PyErr_Format(PyExc_TypeError,
+                         "cannot create '%s' instances", subtype->tp_name);
+            return NULL;
+        }
         PyErr_Format(PyExc_TypeError,
                      "%s.__new__(%s) is not safe, use %s.__new__()",
                      type->tp_name,