gh-142589: Fix PyUnstable_Object_IsUniqueReferencedTemporary

Lib/test/test_capi/test_object.py

@@ -251,6 +251,13 @@ def func(x):
 
         func(object())
 
+        # Test that a newly created object in C is not considered
+        # a uniquely referenced temporary, because it's not on the stack.
+        # gh-142586: do the test in a loop over a list to test for handling
+        # tagged ints on the stack.
+        for i in [0, 1, 2]:
+            self.assertFalse(_testcapi.pyobject_is_unique_temporary_new_object())
+
     def pyobject_dump(self, obj, release_gil=False):
         pyobject_dump = _testcapi.pyobject_dump
 

Misc/NEWS.d/next/C_API/2025-12-11-13-01-49.gh-issue-142589.nNAqgw.rst

@@ -0,0 +1,2 @@
+Fix :c:func:`PyUnstable_Object_IsUniqueReferencedTemporary()` handling of
+tagged ints on the interpreter stack.

Modules/_testcapi/object.c

@@ -138,6 +138,15 @@ pyobject_is_unique_temporary(PyObject *self, PyObject *obj)
     return PyLong_FromLong(result);
 }
 
+static PyObject *
+pyobject_is_unique_temporary_new_object(PyObject *self, PyObject *unused)
+{
+    PyObject *obj = PyList_New(0);
+    int result = PyUnstable_Object_IsUniqueReferencedTemporary(obj);
+    Py_DECREF(obj);
+    return PyLong_FromLong(result);
+}
+
 static int MyObject_dealloc_called = 0;
 
 static void
@@ -517,6 +526,7 @@ static PyMethodDef test_methods[] = {
     {"pyobject_clear_weakrefs_no_callbacks", pyobject_clear_weakrefs_no_callbacks, METH_O},
     {"pyobject_enable_deferred_refcount", pyobject_enable_deferred_refcount, METH_O},
     {"pyobject_is_unique_temporary", pyobject_is_unique_temporary, METH_O},
+    {"pyobject_is_unique_temporary_new_object", pyobject_is_unique_temporary_new_object, METH_NOARGS},
     {"test_py_try_inc_ref", test_py_try_inc_ref, METH_NOARGS},
     {"test_xincref_doesnt_leak",test_xincref_doesnt_leak,        METH_NOARGS},
     {"test_incref_doesnt_leak", test_incref_doesnt_leak,         METH_NOARGS},

Objects/object.c

@@ -2759,8 +2759,12 @@ PyUnstable_Object_IsUniqueReferencedTemporary(PyObject *op)
     _PyStackRef *stackpointer = frame->stackpointer;
     while (stackpointer > base) {
         stackpointer--;
-        if (op == PyStackRef_AsPyObjectBorrow(*stackpointer)) {
-            return PyStackRef_IsHeapSafe(*stackpointer);
+        _PyStackRef ref = *stackpointer;
+        if (PyStackRef_IsTaggedInt(ref)) {
+            continue;
+        }
+        if (op == PyStackRef_AsPyObjectBorrow(ref)) {
+            return PyStackRef_IsHeapSafe(ref);
         }
     }
     return 0;