Define `_Py_TYPEOF` on MSVC and other compilers supporting `__typeof__`

[fuhsnn]

Feature or enhancement

Proposal:

Recently, MSVC implemented __typeof__, which currently is defined as _Py_TYPEOF only on GCC and Clang.

cpython/Include/pyport.h

Lines 553 to 560 in 65feded

	// _Py_TYPEOF(expr) gets the type of an expression.
	//
	// Example: _Py_TYPEOF(x) x_copy = (x);
	//
	// The macro is only defined if GCC or clang compiler is used.
	#if defined(__GNUC__) || defined(__clang__)
	# define _Py_TYPEOF(expr) __typeof__(expr)
	#endif

The only users of _Py_TYPEOF are macros Py_CLEAR(), Py_SETREF() and Py_XSETREF().
If _Py_TYPEOF is not defined, these macros fall back to slightly uglier implementations with memcpy calls.

cpython/Include/refcount.h

Lines 421 to 442 in 65feded

	#ifdef _Py_TYPEOF
	#define Py_CLEAR(op) \
	do { \
	_Py_TYPEOF(op)* _tmp_op_ptr = &(op); \
	_Py_TYPEOF(op) _tmp_old_op = (*_tmp_op_ptr); \
	if (_tmp_old_op != NULL) { \
	*_tmp_op_ptr = _Py_NULL; \
	Py_DECREF(_tmp_old_op); \
	} \
	} while (0)
	#else
	#define Py_CLEAR(op) \
	do { \
	PyObject **_tmp_op_ptr = _Py_CAST(PyObject**, &(op)); \
	PyObject *_tmp_old_op = (*_tmp_op_ptr); \
	if (_tmp_old_op != NULL) { \
	PyObject *_null_ptr = _Py_NULL; \
	memcpy(_tmp_op_ptr, &_null_ptr, sizeof(PyObject*)); \
	Py_DECREF(_tmp_old_op); \
	} \
	} while (0)
	#endif

The proposal is to define _Py_TYPEOF on:

• latest MSVC versions that support __typeof__
• arbitrary compilers that support __typeof__
• arbitrary compilers that support C23 typeof or C++11 decltype

By defining _Py_TYPEOF, uses of Py_CLEAR(), Py_SETREF() and Py_XSETREF() are switched to the _Py_TYPEOF implementation; on these compilers, the benefits would be:

• Less worrying about miscompilation and machine code quality.
• Fewer surprises with hidden memcpy as seen in
• • gh-98724: Fix the Py_CLEAR() macro in the limited C API #100121
• • memcpy() usage without #include <string.h> in pwdmodule.c #121617
• Reduced variance between optimization settings thanks to more straightforward implementation.

Additionally, this removes a differing detail between MSVC and GCC/Clang builds, which should be generally desirable.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

• Macro Py_CLEAR references argument two times. #98724

Linked PRs

• gh-121647: Define _Py_TYPEOF macro on more compilers #121648

[sergey-miryanov]

It seems to have been added