Apply suggestions from code review

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: serhiy-storchaka

Lib/test/test_descr.py

@@ -1384,6 +1384,14 @@ class W(_testcapi.HeapCCollection):
             a.foo = 42
         self.assertIs(weakref.ref(a)(), a)
 
+        with self.assertRaises(TypeError):
+            class X(_testcapi.HeapCCollection):
+                __slots__ = ['x']
+
+        with self.assertRaises(TypeError):
+            class X(_testcapi.HeapCCollection):
+                __slots__ = ['__dict__', 'x']
+
     @support.subTests(('base', 'arg'), [
         (tuple, (1, 2, 3)),
         (int, 9876543210**2),
@@ -1414,6 +1422,13 @@ class W(base):
         self.assertIs(weakref.ref(a)(), a)
         self.assertEqual(a, base(arg))
 
+        with self.assertRaises(TypeError):
+            class X(base):
+                __slots__ = ['x']
+        with self.assertRaises(TypeError):
+            class X(base):
+                __slots__ = ['__dict__', 'x']
+
     def test_slots_special2(self):
         # Testing __qualname__ and __classcell__ in __slots__
         class Meta(type):

Objects/typeobject.c

@@ -4643,7 +4643,7 @@ type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type, PyObject *dict
         Py_ssize_t nslot = PyTuple_GET_SIZE(et->ht_slots);
         if (ctx->base->tp_itemsize != 0) {
             PyErr_Format(PyExc_TypeError,
-                         "nonempty __slots__ not supported for subtype of '%s'",
+                         "arbitrary __slots__ not supported for subtype of '%s'",
                          ctx->base->tp_name);
             return -1;
         }