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

[Eric Abrahamsen]

Noam Postavsky <npostavs@users.sourceforge.net> writes:

> Eric Abrahamsen <eric@ericabrahamsen.net> writes:
>
>> (defclass person ()
>>   ((name :type string :initarg :name)))
>>
>> (defclass classy (eieio-persistent)
>>   ((teacher
>>     :type person
>>     :initarg :teacher)
>>    (students
>>     :initarg :students :initform (make-hash-table))))
>>
>> (setq jane (make-instance 'person :name "Jane"))
>> (setq bob (make-instance 'person :name "Bob"))
>>
>> (setq class (make-instance 'classy
>>                         :teacher jane))
>> (puthash "Bob" bob (slot-value class 'students))
>
>> Jane is `read'-able, Bob isn't. `object-write' will recurse and use
>> `object-write' on any member objects, but the hashtable gets sent to
>> `prin1', which I assume uses `prin1' on all its values instead:
>
> Yes, but this is the case in emacs-25 as well.  The significant
> difference seems to be in the class object:
>
> 25.3:
> (eieio--object-class-tag bob) => eieio-class-tag--person
> 26:
> (eieio--object-class-tag bob) => #2=#s(eieio--class person nil nil
> [#s(cl-slot-descriptor name unbound string nil)] #s(hash-table size 65
> test eq rehash-size 1.5 rehash-threshold 0.8125 data (name 0)) nil
> ((:name . name)) #1=[] #1# #s(#2# unbound) (:custom-groups nil))
>
> In Emacs 25 we have a simple symbol which will round-trip as an `eq'
> object, in Emacs 26 we have a structure which will not (and furthermore
> has some circularity, requiring print-circle to print readably).

Yup, I sure noticed that change when it happened -- it made the already
verbose object representations well-nigh unreadable. Looks like it was
8e6f204f44b. I don't know what the right solution is.