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

[Noam Postavsky <npostavs@users.sourceforge.net>]

pierre.techoueyres@free.fr (Pierre Téchoueyres) writes:

>> 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).
>>
>
> I've tried to dive into print.c but I don't understand how you return
> from print_object, when you're on an hash-table to obtain the vector
> with an eieio-class-tag--<class> (as in 25.3) ?
>
> Can anyone tell me how this work in 25.3 ?

Nothing has changed in print.c in this respect.  In 25.3, objects are
vectors, and class tags are symbols.  In 26, objects are records and
class tags are eieio--class records.  As a result, they print
differently.