gh-87613: Name the subclass in Argument Clinic __new__ argument errors

tp_new is inherited, so argument errors from a subclass named the base
type. While `tp_name` would be slightly faster to use it includes the
module name like `_thread.RLock()` which adds confusion. Use
`_PyType_Name(type)` which returns `RLock()`. Include a test to check.

_PyArg_NoPositional() only short-circuits when args is NULL, which
tp_new never gets; use _PyArg_NoPositionalStack() to keep the name
lookup off the success path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: cmaloney

Lib/test/clinic.test.c

@@ -4968,7 +4968,7 @@ Test___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 
     if ((Py_IS_TYPE(self, base_tp) ||
          Py_TYPE(self)->tp_new == base_tp->tp_new) &&
-        !_PyArg_NoPositional("Test", args)) {
+        !_PyArg_NoPositionalStack("Test", PyTuple_GET_SIZE(args))) {
         goto exit;
     }
     if ((Py_IS_TYPE(self, base_tp) ||
@@ -5017,7 +5017,7 @@ Test_vectorcall(PyObject *type, PyObject *const *args,
 
 static int
 Test___init___impl(TestObj *self)
-/*[clinic end generated code: output=ce04b9d7beacd894 input=645fe693f6c6b9b6]*/
+/*[clinic end generated code: output=62017ea6333c9634 input=645fe693f6c6b9b6]*/
 
 
 /*[clinic input]
@@ -5042,11 +5042,11 @@ Test(PyTypeObject *type, PyObject *args, PyObject *kwargs)
     PyTypeObject *base_tp = TestType;
 
     if ((type == base_tp || type->tp_init == base_tp->tp_init) &&
-        !_PyArg_NoPositional("Test", args)) {
+        !_PyArg_NoPositionalStack(_PyType_Name(_PyType_CAST(type)), PyTuple_GET_SIZE(args))) {
         goto exit;
     }
     if ((type == base_tp || type->tp_init == base_tp->tp_init) &&
-        !_PyArg_NoKeywords("Test", kwargs)) {
+        !_PyArg_NoKeywords(_PyType_Name(_PyType_CAST(type)), kwargs)) {
         goto exit;
     }
     return_value = Test_impl(type);
@@ -5057,7 +5057,7 @@ Test(PyTypeObject *type, PyObject *args, PyObject *kwargs)
 
 static PyObject *
 Test_impl(PyTypeObject *type)
-/*[clinic end generated code: output=68a117adc057940f input=6fe98a19f097907f]*/
+/*[clinic end generated code: output=deab914e0af675ba input=6fe98a19f097907f]*/
 
 
 /*[clinic input]
@@ -5248,7 +5248,7 @@ Test(PyTypeObject *type, PyObject *args, PyObject *kwargs)
     PyObject *__clinic_args = NULL;
 
     if ((type == base_tp || type->tp_init == base_tp->tp_init) &&
-        !_PyArg_NoKeywords("Test", kwargs)) {
+        !_PyArg_NoKeywords(_PyType_Name(_PyType_CAST(type)), kwargs)) {
         goto exit;
     }
     __clinic_args = Py_NewRef(args);
@@ -5263,7 +5263,7 @@ Test(PyTypeObject *type, PyObject *args, PyObject *kwargs)
 
 static PyObject *
 Test_impl(PyTypeObject *type, PyObject *args)
-/*[clinic end generated code: output=e6fba0c8951882fd input=8ce30adb836aeacb]*/
+/*[clinic end generated code: output=9e97b89fb3f12f76 input=8ce30adb836aeacb]*/
 
 
 /*[clinic input]

Lib/test/test_clinic.py

@@ -5806,6 +5806,17 @@ def test_vc_new_base_subclass(self):
         self.assertIsInstance(obj, Sub)
         self.assertIsInstance(obj, ac_tester.VcNewBase)
 
+    def test_new_errors_name_subclass(self):
+        # tp_new is inherited, so its errors name the subclass; the
+        # vectorcall only sees the base type.
+        Sub = type('Sub', (float,), {})
+        with self.assertRaisesRegex(TypeError, r'^float expected at most 1'):
+            float(1, 2)
+        with self.assertRaisesRegex(TypeError, r'^Sub expected at most 1'):
+            Sub(1, 2)
+        with self.assertRaisesRegex(TypeError, r'^Sub\(\) takes no keyword'):
+            Sub(x=1)
+
     def test_vc_kwonly(self):
         # keyword-only 'b': vectorcall has no kwnames==NULL fast path,
         # so every call goes through the helper.

Misc/NEWS.d/next/Tools-Demos/2026-10-27-19-26-26.gh-issue-87613.B-mZEA.rst

@@ -0,0 +1,2 @@
+Generate subclass names in Argument Clinic ``__new__`` error messages rather
+than hardcoding the base type.