pythongh-135906: Use _PyObject_CAST in internal headers (pythonGH-135892)

Fixes build errors encountered in python-greenlet/greenlet#450 when building greenlet on the free-threaded build.

---------

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 3 people

Include/internal/pycore_object.h

@@ -614,7 +614,7 @@ static inline PyObject *
 _Py_XGetRef(PyObject **ptr)
 {
     for (;;) {
-        PyObject *value = _Py_atomic_load_ptr(ptr);
+        PyObject *value = _PyObject_CAST(_Py_atomic_load_ptr(ptr));
         if (value == NULL) {
             return value;
         }
@@ -629,7 +629,7 @@ _Py_XGetRef(PyObject **ptr)
 static inline PyObject *
 _Py_TryXGetRef(PyObject **ptr)
 {
-    PyObject *value = _Py_atomic_load_ptr(ptr);
+    PyObject *value = _PyObject_CAST(_Py_atomic_load_ptr(ptr));
     if (value == NULL) {
         return value;
     }

Include/internal/pycore_stackref.h

@@ -829,7 +829,7 @@ _Py_TryIncrefCompareStackRef(PyObject **src, PyObject *op, _PyStackRef *out)
 static inline int
 _Py_TryXGetStackRef(PyObject **src, _PyStackRef *out)
 {
-    PyObject *op = _Py_atomic_load_ptr_relaxed(src);
+    PyObject *op = _PyObject_CAST(_Py_atomic_load_ptr_relaxed(src));
     if (op == NULL) {
         *out = PyStackRef_NULL;
         return 1;

Misc/NEWS.d/next/C_API/2025-06-25-01-03-10.gh-issue-135906.UBrCWq.rst

@@ -0,0 +1 @@
+Fix compilation errors when compiling the internal headers with a C++ compiler.