bpo-30459: Cast the result of PyCell_SET to void (GH-23654)

Author: vstinner

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
 ----------
 

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
 }

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.