From 0ef96c2b2a291c9d2d9c0ba42bbc1900a21e65f3 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Mon, 7 Dec 2020 11:56:20 +0100
Subject: [PATCH] bpo-30459: Cast the result of PyCell_SET to void (GH-23654)

---
 Doc/whatsnew/3.10.rst                                     | 7 +++++++
 Include/cellobject.h                                      | 2 +-
 .../next/C API/2020-05-06-23-54-57.bpo-30459.N9_Jai.rst   | 8 ++++++--
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index 019dd1817d2..a5cb4e30616 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -611,6 +611,13 @@ Porting to Python 3.10
   :ref:`Python Path Configuration.  <init-path-config>`.
   (Contributed by Victor Stinner in :issue:`42260`.)
 
+* :c:func:`PyList_SET_ITEM`, :c:func:`PyTuple_SET_ITEM` and
+  :c:func:`PyCell_SET` macros can no longer be used as l-value or r-value.
+  For example, ``x = PyList_SET_ITEM(a, b, c)`` and
+  ``PyList_SET_ITEM(a, b, c) = x`` now fail with a compiler error. It prevents
+  bugs like ``if (PyList_SET_ITEM (a, b, c) < 0) ...`` test.
+  (Contributed by Zackery Spytz and Victor Stinner in :issue:`30459`.)
+
 Deprecated
 ----------
 
diff --git a/Include/cellobject.h b/Include/cellobject.h
index f12aa90a42a..81bc784d36f 100644
--- a/Include/cellobject.h
+++ b/Include/cellobject.h
@@ -20,7 +20,7 @@ PyAPI_FUNC(PyObject *) PyCell_Get(PyObject *);
 PyAPI_FUNC(int) PyCell_Set(PyObject *, PyObject *);
 
 #define PyCell_GET(op) (((PyCellObject *)(op))->ob_ref)
-#define PyCell_SET(op, v) (((PyCellObject *)(op))->ob_ref = v)
+#define PyCell_SET(op, v) ((void)(((PyCellObject *)(op))->ob_ref = v))
 
 #ifdef __cplusplus
 }
diff --git a/Misc/NEWS.d/next/C API/2020-05-06-23-54-57.bpo-30459.N9_Jai.rst b/Misc/NEWS.d/next/C API/2020-05-06-23-54-57.bpo-30459.N9_Jai.rst
index e3ee6dccdaa..092d457855a 100644
--- a/Misc/NEWS.d/next/C API/2020-05-06-23-54-57.bpo-30459.N9_Jai.rst	
+++ b/Misc/NEWS.d/next/C API/2020-05-06-23-54-57.bpo-30459.N9_Jai.rst	
@@ -1,2 +1,6 @@
-Cast the result of :c:func:`PyList_SET_ITEM` and :c:func:`PyTuple_SET_ITEM`
-to void.
+:c:func:`PyList_SET_ITEM`, :c:func:`PyTuple_SET_ITEM` and :c:func:`PyCell_SET`
+macros can no longer be used as l-value or r-value.  For example,
+``x = PyList_SET_ITEM(a, b, c)`` and ``PyList_SET_ITEM(a, b, c) = x`` now fail
+with a compiler error. It prevents bugs like
+``if (PyList_SET_ITEM (a, b, c) < 0) ...`` test.
+Patch by Zackery Spytz and Victor Stinner.