On Aug 6, 2016, at 00:04, Guido van Rossum <guido@python.org> wrote:
> Different question. What features are we actually talking about? Is it
> possible to enumerate them?
>
> The only thing I'm aware of is declarations following non-declarations
> in the same block, but I'm no C expert any more.

I'm certainly not, either.  Looking back over the original thread, there were a number of features talked about.  I think Larry's list here captures most of them:

"Clearly it'll be 3.5+ only, and clearly it'll be a specific list of
features ("C89 but also permitting //-comments, variadic macros,
variable declarations on any line, inline functions, and designated
initializers").  But I'm looking forward to it!"

https://mail.python.org/pipermail/python-dev/2016-June/144831.html

So, if that set of features seems reasonable, perhaps the next step is for someone (not me) to put together a test patch that incorporates the use of them and then we can run it against the buildbots, either by using the special build buildbots or just temporarily checking the patch in.  I'm willing to do the testing and results-gathering if a patch is supplied.