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.

Author: vstinner

Include/cpython/object.h

@@ -352,7 +352,7 @@ PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
 #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); \
@@ -374,7 +374,7 @@ 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); \

Include/pyport.h

@@ -538,15 +538,18 @@ extern "C" {
 //
 // Example: _Py_TYPEOF(x) x_copy = (x);
 //
-// On C23, use typeof(). Otherwise __typeof__() if on GCC, clang or
-// MSVC 17.9 and newer. Else if on C++11 or newer, decltype() is used.
+// On C23, use typeof(). On C++11, use decltype(). 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.
 #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)
 #  define _Py_TYPEOF(expr) __typeof__(expr)
-#elif defined(__cplusplus) && __cplusplus >= 201103L
-#  define _Py_TYPEOF(expr) decltype(expr)
 #endif
 
 

Include/refcount.h

@@ -482,7 +482,7 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
 #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; \