Feature or enhancement

Proposal:

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

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.

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
• gh-121647: Prefer decltype() for _Py_TYPEOF() on C++ #157267