Eric Abrahamsen <eric@ericabrahamsen.net> writes:

>>> Bah, this is annoyingly difficult -- thank you for trying it out. I
>>> suppose one terrible heuristic would be if the errors were different
>>> between emacs-25, emacs-26, and fix/eieio-persistent. Would you mind
>>> sending me the error output, in case anything jumps out at me?
>>>
>> Hope this could help you.
>
> Thanks. They all end in "Corrupt object on disk", which is an
> eieio-persist problem. I'll keep working on this.

So, I think it would be nice to have this fixed for 26.3.  It sounds
like the fix you're working on is too risky to have on a release branch
regardless of its current status, so here's a patch which makes eieio
objects use symbols as type tags, when eieio-backward-compatibility is
non-nil.  Folks who want the record-with-circular-references as type
tags can still get that by setting eieio-backward-compatibility to nil.