[3.15] gh-157649: Fix Py_CLEAR()/Py_SETREF() on C++ (#159143)

* gh-121647: Fix _Py_TYPEOF() on C++ (#157267)

Using MSVC (on Windows), _Py_TYPEOF() prefers decltype()
over __typeof__() on C++.

Fix also _Py_TYPEOF() usage in Py_CLEAR() and Py_SETREF() macros.
Replace "_Py_TYPEOF(dst)*" with "_Py_TYPEOF(&(dst))". In C++,
"_Py_TYPEOF(dst)*" can fail with a compiler error.

(cherry picked from commit bba99f2)

* 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.

(cherry picked from commit 3330712)

Author: vstinner

Include/cpython/object.h

@@ -340,21 +340,29 @@ 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'.
  */
 #ifdef _Py_TYPEOF
 #define Py_SETREF(dst, src) \
     do { \
-        _Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
+        _Py_TYPEOF(&(dst)) _tmp_dst_ptr = &(dst); \
         _Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
         *_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 { \
@@ -372,11 +380,19 @@ PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
 #ifdef _Py_TYPEOF
 #define Py_XSETREF(dst, src) \
     do { \
-        _Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
+        _Py_TYPEOF(&(dst)) _tmp_dst_ptr = &(dst); \
         _Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
         *_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 { \

Include/pyport.h

@@ -538,11 +538,15 @@ extern "C" {
 //
 // Example: _Py_TYPEOF(x) x_copy = (x);
 //
-// On C23, use typeof(). Otherwise, the macro is only defined
-// if GCC or clang compiler is used.
+// On C23, use typeof(). Otherwise, use __typeof__()
+// if on GCC, clang or MSVC 17.9 and newer.
+//
+// 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(__GNUC__) || defined(__clang__)
+#elif (defined(__GNUC__) || defined(__clang__) \
+       || (defined(_MSC_VER) && _MSC_VER >= 1939 && !defined(__cplusplus)))
 #  define _Py_TYPEOF(expr) __typeof__(expr)
 #endif
 

Include/refcount.h

@@ -478,17 +478,30 @@ 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) \
     do { \
-        _Py_TYPEOF(op)* _tmp_op_ptr = &(op); \
+        _Py_TYPEOF(&(op)) _tmp_op_ptr = &(op); \
         _Py_TYPEOF(op) _tmp_old_op = (*_tmp_op_ptr); \
         if (_tmp_old_op != _Py_NULL) { \
             *_tmp_op_ptr = _Py_NULL; \
             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 { \

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.