bpo-47164: Add _PyCFunction_CAST() macro (GH-32192)

Use the macro in C files of the Python/ directory.

Include/methodobject.h

@@ -26,6 +26,24 @@ typedef PyObject *(*_PyCFunctionFastWithKeywords) (PyObject *,
 typedef PyObject *(*PyCMethod)(PyObject *, PyTypeObject *, PyObject *const *,
                                size_t, PyObject *);
 
+// Cast an function to the PyCFunction type to use it with PyMethodDef.
+//
+// This macro can be used to prevent compiler warnings if the first parameter
+// uses a different pointer type than PyObject* (ex: METH_VARARGS and METH_O
+// calling conventions).
+//
+// The macro can also be used for METH_FASTCALL and METH_VARARGS|METH_KEYWORDS
+// calling conventions to avoid compiler warnings because the function has more
+// than 2 parameters. The macro first casts the function to the
+// "void func(void)" type to prevent compiler warnings.
+//
+// If a function is declared with the METH_NOARGS calling convention, it must
+// have 2 parameters. Since the second parameter is unused, Py_UNUSED() can be
+// used to prevent a compiler warning. If the function has a single parameter,
+// it triggers an undefined behavior when Python calls it with 2 parameters
+// (bpo-33012).
+#define _PyCFunction_CAST(func) ((PyCFunction)(void(*)(void))(func))
+
 PyAPI_FUNC(PyCFunction) PyCFunction_GetFunction(PyObject *);
 PyAPI_FUNC(PyObject *) PyCFunction_GetSelf(PyObject *);
 PyAPI_FUNC(int) PyCFunction_GetFlags(PyObject *);