GeezerVison

PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst));
PyObject *_tmp_old_dst = (*_tmp_dst_ptr);

#include <Python.h>

#include <cstdio>
#include <cstring>

#ifndef _Py_CAST
#  define _Py_CAST(type, expr) ((type)(expr))
#endif
#ifndef _PyObject_CAST
#  define _PyObject_CAST(obj) _Py_CAST(PyObject *, (obj))
#endif

#define REPRO_XSETREF(dst, src) \
    do { \
        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject **, &(dst)); \
        PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
        PyObject *_tmp_src = _PyObject_CAST(src); \
        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject *)); \
        Py_XDECREF(_tmp_old_dst); \
    } while (0)

__attribute__((noinline))
static void
replace(PyTypeObject **a, PyTypeObject **b,
        PyTypeObject *first, PyTypeObject *second)
{
    *a = first;
    REPRO_XSETREF(*b, second);
}

int
main()
{
    Py_Initialize();

    PyType_Slot slots[] = {{0, NULL}};
    PyType_Spec spec = {
        "repro.T", sizeof(PyObject), 0, Py_TPFLAGS_DEFAULT, slots
    };
    PyObject *first = PyType_FromSpec(&spec);
    PyObject *second = PyType_FromSpec(&spec);
    if (first == NULL || second == NULL) {
        PyErr_Print();
        return 1;
    }

    Py_ssize_t before = Py_REFCNT(first);
    PyTypeObject *slot = NULL;

    // Both pointer arguments alias. The assignment makes first the old value
    // that REPRO_XSETREF() must release before storing second.
    replace(&slot, &slot,
            reinterpret_cast<PyTypeObject *>(Py_NewRef(first)),
            reinterpret_cast<PyTypeObject *>(Py_NewRef(second)));

    std::printf("first refcount: %zd; expected: %zd\n",
                Py_REFCNT(first), before);
    return 0;
}

$ g++ -std=c++11 $(python3.10-config --cflags) \
      -DPy_LIMITED_API=0x030a0000 -O2 -fstrict-aliasing \
      repro.cpp $(python3.10-config --embed --ldflags) -o repro
$ ./repro
first refcount: 3; expected: 2

#ifndef Py_SETREF
#define Py_SETREF(dst, src) \
    do { \
        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
        PyObject *_tmp_old_dst; \
        memcpy(&_tmp_old_dst, _tmp_dst_ptr, sizeof(PyObject*)); \
        PyObject *_tmp_src = _PyObject_CAST(src); \
        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
        Py_DECREF(_tmp_old_dst); \
    } while (0)
#endif
#ifndef Py_XSETREF
#define Py_XSETREF(dst, src) \
    do { \
        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
        PyObject *_tmp_old_dst; \
        memcpy(&_tmp_old_dst, _tmp_dst_ptr, sizeof(PyObject*)); \
        PyObject *_tmp_src = _PyObject_CAST(src); \
        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
        Py_XDECREF(_tmp_old_dst); \
    } while (0)
#endif