gh-78465: Fix error message for cls.__new__(cls, ...) where cls is not instantiable (GH-135981)

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

Lib/test/support/__init__.py

@@ -2333,6 +2333,7 @@ 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

@@ -869,12 +869,7 @@ class SysModuleTest(unittest.TestCase):
     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

@@ -2,7 +2,7 @@
 
 from test.support import (
     run_with_locale, cpython_only, no_rerun,
-    MISSING_C_DOCSTRINGS, EqualToForwardRef,
+    MISSING_C_DOCSTRINGS, EqualToForwardRef, check_disallow_instantiation,
 )
 from test.support.script_helper import assert_python_ok
 from test.support.import_helper import import_fresh_module
@@ -1148,8 +1148,7 @@ class UnionTests(unittest.TestCase):
                              msg='Check for union reference leak.')
 
     def test_instantiation(self):
-        with self.assertRaises(TypeError):
-            types.UnionType()
+        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])

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

@@ -10020,6 +10020,11 @@ 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,