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Argument Clinic no longer works single optional argument. For example see _tkinter.tkapp.wantobjects in the patch in issue20168.
/*[clinic input]
_tkinter.tkapp.wantobjects
 [
 value: int
 ]
 /
[clinic start generated code]*/
It is converted to the methoddef of type METH_O.
#define _TKINTER_TKAPP_WANTOBJECTS_METHODDEF \
 {"wantobjects", (PyCFunction)_tkinter_tkapp_wantobjects, METH_O, _tkinter_tkapp_wantobjects__doc__},
static PyObject *
_tkinter_tkapp_wantobjects(PyObject *self, PyObject *arg)
{
 PyObject *return_value = NULL;
 int group_right_1 = 0;
 int value = 0;
 if (!PyArg_Parse(arg, "i:wantobjects", &value))
 goto exit;
 return_value = _tkinter_tkapp_wantobjects_impl(self, group_right_1, value);
exit:
 return return_value;
}
As result wantobjects() can't be called without an argument.

Optional groups were designed to support two specific legacy use cases:
 * functions that have groups of parameters that are 
 * functions with optional parameters to the left of required parameters
They are not intended to be used in cases like this. I really, really wish you would stop using them except for these specific legacy cases.
Furthermore, the argument parsing for tkinter.tkapp.wantobjects has a very clear translation into Argument Clinic:
 value: int = -1
 /
Optional groups should not be used for single-argument functions. I have no interest in fixing this bug.

Sorry, I forgot to finish the first bullet point. It should have read:
 * functions that have groups of parameters that are optional only as a group (e.g. curses.window.getch)

Then it would be better to not convert wantobjects() to Argument Clinic. Using -1 as a value for not passed argument is a hack (added in 1ab9978123be) and I don't want to propagate it to the signature.
But I afraid that there are other functions that suffer from this bug (may be in the curses module).

If the argument currently uses a default value of -1, then I see no problem with converting it to Argument Clinic using a default value of -1. If you claim it's a "hack" then you should discuss that with the author of 1ab9978123be.

Closing as wontfix. This is not a supported use of optional groups.