[PyROOT] Example with inheriting from ROOT.Math.IMultiGenFunction doesn't work after recent cppyy upgrade

[guitargeek]

Description

One example from the ROOT manual doesn't work anymore with ROOT 6.32:
https://root.cern/manual/python/#alternative-for-tpymultigenfunction-and-tpymultigradfunction

import ROOT

from array import array

class MyMultiGenFCN( ROOT.Math.IMultiGenFunction ):
    def NDim( self ):
        return 1

    def DoEval( self, x ):
        return (x[0] - 42) * (x[0] - 42)

    def Clone( self ):
        x = MyMultiGenFCN()
        ROOT.SetOwnership(x, False)
        return x

def main():
    fitter = ROOT.Fit.Fitter()
    myMultiGenFCN = MyMultiGenFCN()
    params = array('d', [1.])
    fitter.FitFCN(myMultiGenFCN, params)
    fitter.Result().Print(ROOT.std.cout, True)

if __name__ == '__main__':
    main()

The error is:

input_line_33:38:23: error: expected '(' for function-style cast or type construction
      return unsigned int{};
             ~~~~~~~~ ^
Traceback (most recent call last):
  File "/home/rembserj/example.py", line 5, in <module>
    class MyMultiGenFCN( ROOT.Math.IMultiGenFunction ):
TypeError: no python-side overrides supported (failed to compile the dispatcher code)

See:

• https://root.cern/manual/python/#alternative-for-tpymultigenfunction-and-tpymultigradfunction
• https://root-forum.cern.ch/t/minuit2minimizer-with-root-math-imultigenfunction/59102

[aaronj0]

For the IMultiGenFunction crossinheritance I see that we inject theNDim() method into the dispatched code:

    PyObject* iself = (PyObject*)_internal_self;
    if (!iself || iself == Py_None) {
      PyErr_Warn(PyExc_RuntimeWarning, (char*)"Call attempted on deleted python-side proxy");
      return unsigned int{};
    }

using this code block:

root/bindings/pyroot/cppyy/CPyCppyy/src/Dispatcher.cxx

Lines 41 to 51 in 0aa2b6b

	// possible crash
	code << " PyObject* iself = (PyObject*)_internal_self;\n"
	" if (!iself || iself == Py_None) {\n"
	" PyErr_Warn(PyExc_RuntimeWarning, (char*)\"Call attempted on deleted python-side proxy\");\n"
	" return";
	if (retType != "void") {
	if (retType.back() != '*')
	code << " " << CPyCppyy::TypeManip::remove_const(retType) << "{}";
	else
	code << " nullptr";
	}

Cling fails to compile the initialization in return unsigned int{}; since it is not valid C++. We should also consider unsigned int and other multi keyword types that require a function style cast for value initialization (like return (unsigned int){}. If I am not wrong, this should just work for all primitives(as zero initialization) and user-defined types(as default initialization).

I was able to get this example from the manual to work with this fix. Based on the issue, I was of the opinion we should land this upstream but I see that this is a high priority issue and @wlav is on vacation.

What do you think @guitargeek @dpiparo