[issue28933] AC: Accept None as a Py_ssize_t default value

[Julien Palard]

Julien Palard added the comment:

> You propose an automatic conversion of "None" into "-1"?  That's awful.  Please don't commit that patch to CPython.

Not really, I propose a way to do it with AC when needed. And the AC semantics introduced are not "Automatic conversion of None into -1" but "automatic "don't touch the C default" when the python default is given", so it's reusable for other values:

    something: Py_ssize_t(c_default="-1") = None
    something: Py_ssize_t(c_default="0") = None
    something: Py_ssize_t(c_default="42") = None
    …

And this semantic can be extended to other types too if needed:

    something: a_type(c_default="a_c_side_default_value") = None

To get "a_c_side_default_value" when None is given.

I however did not introduced a way to keep the C default value by using something else than None in the Python side:

 - I'm not sure this is usefull to allow it but it can still be implemented if needed
 - Limiting to None permits to keep a type check so ``something: Py_ssize_t(c_default="-1") = "bar"`` will still fail at clinic.py runtime.

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<http://bugs.python.org/issue28933>
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