https://github.com/python/cpython/commit/333071231d3a46cccc32d7f44b99328c3299d0b1
commit: 333071231d3a46cccc32d7f44b99328c3299d0b1
branch: main
author: Victor Stinner <[email protected]>
committer: vstinner <[email protected]>
date: 2026-09-28T19:18:28Z
summary:
gh-157649: Fix Py_CLEAR()/Py_SETREF() on C++ (#158070)
On C++, do not use decltype() in _Py_TYPEOF since it produces invalid
code in Py_CLEAR() and Py_SETREF(). Instead, implement Py_CLEAR(),
Py_SETREF() and Py_XSETREF() using "auto" on C++11 and newer.
Add Py_CLEAR(), Py_SETREF() and Py_XSETREF() tests on an array.
files:
A Misc/NEWS.d/next/C_API/2026-09-24-11-47-32.gh-issue-157649.3HSU2h.rst
M Include/cpython/object.h
M Include/pyport.h
M Include/refcount.h
M Lib/test/test_cext/extension.c
diff --git a/Include/cpython/object.h b/Include/cpython/object.h
index c0fcfc363ef90c9..5d35f983d2833c3 100644
--- a/Include/cpython/object.h
+++ b/Include/cpython/object.h
@@ -342,9 +342,9 @@ PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
* `dst` points to a valid object.
*
* Temporary variables are used to only evaluate macro arguments once and so
- * avoid the duplication of side effects. _Py_TYPEOF() or memcpy() is used to
- * avoid a miscompilation caused by type punning. See Py_CLEAR() comment for
- * implementation details about type punning.
+ * avoid the duplication of side effects. _Py_TYPEOF(), C++ auto, or memcpy()
+ * is used to avoid a miscompilation caused by type punning. See Py_CLEAR()
+ * comment for implementation details about type punning.
*
* The memcpy() implementation does not emit a compiler warning if 'src' has
* not the same type than 'src': any pointer type is accepted for 'src'.
@@ -357,6 +357,14 @@ PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
*_tmp_dst_ptr = (src); \
Py_DECREF(_tmp_old_dst); \
} while (0)
+#elif defined(__cplusplus) && (__cplusplus >= 201103L || _MSVC_LANG >=
201103L)
+#define Py_SETREF(dst, src) \
+ do { \
+ auto _tmp_dst_ptr = &(dst); \
+ auto _tmp_old_dst = (*_tmp_dst_ptr); \
+ *_tmp_dst_ptr = (src); \
+ Py_DECREF(_tmp_old_dst); \
+ } while (0)
#else
#define Py_SETREF(dst, src) \
do { \
@@ -379,6 +387,14 @@ PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
*_tmp_dst_ptr = (src); \
Py_XDECREF(_tmp_old_dst); \
} while (0)
+#elif defined(__cplusplus) && (__cplusplus >= 201103L || _MSVC_LANG >=
201103L)
+#define Py_XSETREF(dst, src) \
+ do { \
+ auto _tmp_dst_ptr = &(dst); \
+ auto _tmp_old_dst = (*_tmp_dst_ptr); \
+ *_tmp_dst_ptr = (src); \
+ Py_XDECREF(_tmp_old_dst); \
+ } while (0)
#else
#define Py_XSETREF(dst, src) \
do { \
diff --git a/Include/pyport.h b/Include/pyport.h
index 744bae6c57e299e..9cfdd09689d5c81 100644
--- a/Include/pyport.h
+++ b/Include/pyport.h
@@ -538,17 +538,15 @@ extern "C" {
//
// Example: _Py_TYPEOF(x) x_copy = (x);
//
-// On C23, use typeof(). On C++11, use decltype(). Otherwise, use __typeof__()
+// On C23, use typeof(). Otherwise, use __typeof__()
// if on GCC, clang or MSVC 17.9 and newer.
//
-// On MSVC, check also _MSVC_LANG since __cplusplus is 199711L unless
-// the /Zc:__cplusplus flag is used.
+// gh-157649: Do not use decltype() on C++, since it produces invalid code in
+// Py_CLEAR()/Py_SETREF().
#if defined (__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
# define _Py_TYPEOF(expr) typeof(expr)
-#elif defined(__cplusplus) && (__cplusplus >= 201103L || _MSVC_LANG >=
201103L)
-# define _Py_TYPEOF(expr) decltype(expr)
-#elif defined(__GNUC__) || defined(__clang__) || \
- (defined(_MSC_VER) && _MSC_VER >= 1939)
+#elif (defined(__GNUC__) || defined(__clang__) \
+ || (defined(_MSC_VER) && _MSC_VER >= 1939 && !defined(__cplusplus)))
# define _Py_TYPEOF(expr) __typeof__(expr)
#endif
diff --git a/Include/refcount.h b/Include/refcount.h
index d96c75421aef337..39661680e85a183 100644
--- a/Include/refcount.h
+++ b/Include/refcount.h
@@ -478,6 +478,9 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
* and so avoid type punning. Otherwise, use memcpy() which causes type erasure
* and so prevents the compiler to reuse an old cached 'op' value after
* Py_CLEAR().
+ *
+ * On C++11 and newer, use "auto". On MSVC, check also _MSVC_LANG since
+ * __cplusplus is 199711L unless the /Zc:__cplusplus flag is used.
*/
#ifdef _Py_TYPEOF
#define Py_CLEAR(op) \
@@ -489,6 +492,16 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
Py_DECREF(_tmp_old_op); \
} \
} while (0)
+#elif defined(__cplusplus) && (__cplusplus >= 201103L || _MSVC_LANG >=
201103L)
+#define Py_CLEAR(op) \
+ do { \
+ auto _tmp_op_ptr = &(op); \
+ auto _tmp_old_op = (*_tmp_op_ptr); \
+ if (_tmp_old_op != _Py_NULL) { \
+ *_tmp_op_ptr = _Py_NULL; \
+ Py_DECREF(_tmp_old_op); \
+ } \
+ } while (0)
#else
#define Py_CLEAR(op) \
do { \
diff --git a/Lib/test/test_cext/extension.c b/Lib/test/test_cext/extension.c
index b56c4dbe78a3d85..ea7a1ca3c38a84d 100644
--- a/Lib/test/test_cext/extension.c
+++ b/Lib/test/test_cext/extension.c
@@ -82,6 +82,7 @@ static PyObject *
test_macros(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
{
PyObject *obj, *dict;
+ PyObject *slots[1];
// test Py_BUILD_ASSERT() and Py_BUILD_ASSERT_EXPR()
Py_BUILD_ASSERT(sizeof(int) == sizeof(unsigned int));
@@ -97,16 +98,31 @@ test_macros(PyObject *Py_UNUSED(module), PyObject
*Py_UNUSED(args))
Py_CLEAR(obj);
assert(obj == _Py_NULL);
+ // gh-157649: Test Py_CLEAR() on an array
+ slots[0] = Py_None;
+ Py_CLEAR(slots[0]);
+ assert(slots[0] == _Py_NULL);
+
#ifndef Py_LIMITED_API
// Test Py_SETREF(): use typeof()/__typeof__() if available, or memcpy()
obj = Py_None;
Py_SETREF(obj, _Py_NULL);
assert(obj == _Py_NULL);
+ // gh-157649: Test Py_SETREF() on an array
+ slots[0] = Py_None;
+ Py_SETREF(slots[0], _Py_NULL);
+ assert(slots[0] == _Py_NULL);
+
// Test Py_XSETREF(): use typeof()/__typeof__() if available, or memcpy()
obj = Py_None;
Py_XSETREF(obj, _Py_NULL);
assert(obj == _Py_NULL);
+
+ // gh-157649: Test Py_XSETREF() on an array
+ slots[0] = Py_None;
+ Py_XSETREF(slots[0], _Py_NULL);
+ assert(slots[0] == _Py_NULL);
#endif
// Test that Py_BEGIN_CRITICAL_SECTION is available
diff --git
a/Misc/NEWS.d/next/C_API/2026-09-24-11-47-32.gh-issue-157649.3HSU2h.rst
b/Misc/NEWS.d/next/C_API/2026-09-24-11-47-32.gh-issue-157649.3HSU2h.rst
new file mode 100644
index 000000000000000..92eafbdbe39641c
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2026-09-24-11-47-32.gh-issue-157649.3HSU2h.rst
@@ -0,0 +1,4 @@
+Fix :c:macro:`Py_CLEAR` and :c:macro:`Py_SETREF` macros on C++: implement
+them using ``auto`` instead of ``decltype()``. Using ``decltype()``
+produced invalid code when clearing/setting an array item. Patch by Victor
+Stinner.
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