Reuben Thomas <rrt@sc3d.org> writes:

>
> Given my previous error of reasoning, I'm submitting the above for your consideration before I prepare a patch!

Sorry I didn't get around to looking at this before you wrote a patch,
but I think you're overthinking this.  AFAICT, your first fix (in
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=24745#5) was right.
Here's a patch with some tests, let me know if I missed any cases.