gh-157649: Fix Py_CLEAR()/Py_SETREF() on C++

On C++, do not use decltype() in _Py_TYPEOF since it produces invalid
code in Py_CLEAR() and Py_SETREF(). Instead, implement Py_CLEAR() and
Py_SETREF() using "auto" on C++11 and newer.

Add Py_CLEAR() and Py_SETREF() tests on an array.

Author: vstinner

Include/cpython/object.h

@@ -349,7 +349,15 @@ PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
  * 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
+#if 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)
+#elif defined(_Py_TYPEOF)
 #define Py_SETREF(dst, src) \
     do { \
         _Py_TYPEOF(&(dst)) _tmp_dst_ptr = &(dst); \

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))
 #  define _Py_TYPEOF(expr) __typeof__(expr)
 #endif
 

Include/refcount.h

@@ -478,8 +478,21 @@ 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
+#if 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)
+#elif defined(_Py_TYPEOF)
 #define Py_CLEAR(op) \
     do { \
         _Py_TYPEOF(&(op)) _tmp_op_ptr = &(op); \

Lib/test/test_cext/extension.c

@@ -80,6 +80,7 @@ static int
 _testcext_exec(PyObject *module)
 {
     PyObject *result, *obj;
+    PyObject *slots[1];
 
 #ifdef __STDC_VERSION__
     if (PyModule_AddIntMacro(module, __STDC_VERSION__) < 0) {
@@ -105,11 +106,21 @@ _testcext_exec(PyObject *module)
     Py_CLEAR(obj);
     assert(obj == 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, NULL);
     assert(obj == NULL);
+
+    // gh-157649: Test Py_SETREF() on an array
+    slots[0] = Py_None;
+    Py_SETREF(slots[0], _Py_NULL);
+    assert(slots[0] == _Py_NULL);
 #endif
 
     // Test that Py_BEGIN_CRITICAL_SECTION is available

Lib/test/test_cppext/extension.cpp

@@ -307,16 +307,26 @@ _testcppext_exec(PyObject *module)
     assert(Py_MAX(5, 11) == 11);
     assert(Py_ABS(-5) == 5);
 
-    // Test Py_CLEAR(): use typeof()/__typeof__() if available, or memcpy()
+    // Test Py_CLEAR(): use auto on C++11, or memcpy()
     PyObject *obj = Py_None;
     Py_CLEAR(obj);
     assert(obj == _Py_NULL);
 
+    // gh-157649: Test Py_CLEAR() on an array
+    PyObject *slots[1] = {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()
+    // Test Py_SETREF(): use auto on C++11, 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);
 #endif
 
     // Test that Py_BEGIN_CRITICAL_SECTION is available

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.