"With the pystate_cplusplus.patch I was able to compile both min_example.tar.gz and my actual extension.  So I with your patch, it does work.  Thank you."

Cool! I applied this simple patch instead of trying to write an ugly glue in pyatomic.h between C and C++.

C and C++ simply look to be incompatible for handling atomic types :-/ The good news is that we don't them atomic types for Python extensions, only to compile Python core.

Thanks for the report. I prefer to detect such annoying issue before a ".0" release (3.5.0). FYI it's a regression introduced by the issue #22038 which started to use <stdatomic.h> (but it's now fixed).