gh-150868: Also use exact-type checks for ints in the fast sum() loops

Per review: use PyLong_CheckExact with the specialized exact-int
deallocator in the float and complex fast-sum loops, so int subclasses
take the generic path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: eendebakpt

Lib/test/test_builtin.py

@@ -2241,6 +2241,19 @@ def __del__(self):
         self.assertEqual(sum((F(i) for i in range(5)), 1j), complex(10, 1))
         self.assertEqual(F.count, 5)
 
+    def test_sum_int_subclass(self):
+        # gh-151060: likewise, the float and complex fast-sum loops must not
+        # free an int subclass with the exact-int deallocator.
+        class I(int):
+            count = 0
+            def __del__(self):
+                I.count += 1
+
+        self.assertEqual(sum((I(i) for i in range(5)), 1.0), 11.0)
+        self.assertEqual(I.count, 5)
+        self.assertEqual(sum((I(i) for i in range(5)), 1j), complex(10, 1))
+        self.assertEqual(I.count, 10)
+
     @requires_IEEE_754
     @skip_if_double_rounding
     @support.cpython_only    # Other implementations may choose a different algorithm

Python/bltinmodule.c

@@ -2981,15 +2981,15 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
                 _Py_DECREF_SPECIALIZED(item, _PyFloat_ExactDealloc);
                 continue;
             }
-            if (PyLong_Check(item)) {
+            if (PyLong_CheckExact(item)) {
                 double value = PyLong_AsDouble(item);
                 if (value != -1.0 || !PyErr_Occurred()) {
                     re_sum = cs_add(re_sum, value);
-                    Py_DECREF(item);
+                    _Py_DECREF_SPECIALIZED(item, _PyLong_ExactDealloc);
                     continue;
                 }
                 else {
-                    Py_DECREF(item);
+                    _Py_DECREF_SPECIALIZED(item, _PyLong_ExactDealloc);
                     Py_DECREF(iter);
                     return NULL;
                 }
@@ -3033,15 +3033,15 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
                 Py_DECREF(item);
                 continue;
             }
-            if (PyLong_Check(item)) {
+            if (PyLong_CheckExact(item)) {
                 double value = PyLong_AsDouble(item);
                 if (value != -1.0 || !PyErr_Occurred()) {
                     re_sum = cs_add(re_sum, value);
-                    Py_DECREF(item);
+                    _Py_DECREF_SPECIALIZED(item, _PyLong_ExactDealloc);
                     continue;
                 }
                 else {
-                    Py_DECREF(item);
+                    _Py_DECREF_SPECIALIZED(item, _PyLong_ExactDealloc);
                     Py_DECREF(iter);
                     return NULL;
                 }