The suggested wild-card function type for functions taking 2 or more parameters
would be something like this:

PyObject * (*) (PyObject *, PyObject *, ...);

It is still necessary to cast it back to the correct type before calling.

There's also a PyCFunctionWithoutArgs that takes just one parameter so it'll have to be one arg onwards but I don't know the impact on it's API. I'm going to test that when I get back home tonight and propose a fix in Python. I kinda understand the rationale behind not diluting the warning, but I'd like to keep this open pending conclusion of the discussion on the ML.

Actually the warning on PyCFunctionWithoutArgs
is _not_ a false positive:

I am looking at Python-3.6.1 right now.
What I see is that functions with no arguments
have the signature "PyObject * (*)(PyObject *)"
but on the call side the Call passes two arguments
see Python-3.6.1/Objects/methodobject.c:

        case METH_NOARGS:
            size = PyTuple_GET_SIZE(args);
            if (size != 0) {
                PyErr_Format(PyExc_TypeError,
                    "%.200s() takes no arguments (%zd given)",
                    f->m_ml->ml_name, size);
                return NULL;
            }

            res = (*meth)(self, NULL);

Also these look like more like invalid casts than bogus warnings:

../Python-3.6.1/Objects/frameobject.c:586:5: Warnung: cast between incompatible function types from »void (*)(PyFrameObject *)« {alias »void (*)(struct _fra
me *)«} to »int (*)(PyObject *)« {alias »int (*)(struct _object *)«} [-Wcast-function-type]
     (inquiry)frame_tp_clear,                    /* tp_clear */
     ^
../Python-3.6.1/Objects/funcobject.c:408:18: Warnung: cast between incompatible function types from »PyObject * (*)(PyFunctionObject *)« {alias »struct _obj
ect * (*)(struct <anonym> *)«} to »PyObject * (*)(PyObject *, void *)« {alias »struct _object * (*)(struct _object *, void *)«} [-Wcast-function-type]
     {"__code__", (getter)func_get_code, (setter)func_set_code},
                  ^
../Python-3.6.1/Objects/funcobject.c:408:41: Warnung: cast between incompatible function types from »int (*)(PyFunctionObject *, PyObject *)« {alias »int (*
)(struct <anonym> *, struct _object *)«} to »int (*)(PyObject *, PyObject *, void *)« {alias »int (*)(struct _object *, struct _object *, void *)«} [-Wcast-
function-type]
     {"__code__", (getter)func_get_code, (setter)func_set_code},
                                         ^

And unfortunately the last two would be suppressed by the proposed patch.

Yeah that doesn't look good.  I had not done a cpython build; I had started with a gem5 build (which builds python modules) and only superficially looked at cpython.

I thought about this over the weekend and agree that silencing the warning will only hold us back indefinitely.  I'll look at the cpython build a bit closer and try to fix it there when I have time for it.  Thanks for looking into this.