The interesting discovery I made while reviewing the patch for issue 22906 is that there apparently *is* implicit chaining support in PyErr_SetObject: https://hg.python.org/cpython/file/default/Python/errors.c#l70

Chris indicates that it doesn't seem to be triggering for his patch, though (even after normalising and then restoring the exception state), and I haven't fully dug into why yet. Preliminary explorations show that the last two functional modifications were a fix for a crash bug in issue 3611, and Antoine's original implementation of exception chaining in issue 3108.

I've added Antoine to the nosy list, as my main takeaway at the moment is that I *don't* currently understand what's going on with the exception chaining, and I'd like to before we merge the PEP 479 patch.