bug#29220: 26.0.90; eieio-persistent-read fail to restore saved object.

[Noam Postavsky]

> Subject: [PATCH] Use plain symbols for eieio type descriptors (Bug#29220)
>
> Since Emacs 26, eieio objects use a class record (with circular
> references) as the type descriptor of the object record.  This causes
> problems when reading back an object from a string, because the class
> record is not `eq' to the canonical one (which means that read objects
> don't satisfy the foo-p predicate).
> * lisp/emacs-lisp/eieio.el (make-instance): As a (partial) fix, set
> the record's type descriptor to a plain symbol for the type descriptor
> when eieio-backward-compatibility is non-nil (the default).
> * lisp/emacs-lisp/eieio-core.el (eieio--object-class): Call
> eieio--class-object on the type tag when eieio-backward-compatibility
> is non-nil.
> (eieio-object-p): Use eieio--object-class instead of
> eieio--object-class-tag.

Pushed to emacs-26 (with the tests expanded to show expected fail with
eieio-backward-compatibility set to nil).  I'm leaving the bug open
since this still fails with eieio-backward-compatibility=nil.

5f01af6c8e 2019-05-30T18:46:07-04:00 "Use plain symbols for eieio type 
descriptors (Bug#29220)"
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=5f01af6c8e0f7355f7a99a80ff32369071f65eda