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.
* test/lisp/emacs-lisp/eieio-tests/eieio-test-persist.el
(eieio-test-persist-hash-and-vector)
(eieio-test-persist-interior-lists): Make into functions.
(eieio-persist-hash-and-vector-backward-compatibility)
(eieio-persist-hash-and-vector-no-backward-compatibility)
(eieio-test-persist-interior-lists-backward-compatibility)
(eieio-test-persist-interior-lists-no-backward-compatibility): New
tests which call them, eieio-backward-compatibility let-bound.

1 files changed, 22 insertions, 6 deletions

diff --git a/test/lisp/emacs-lisp/eieio-tests/eieio-test-persist.el b/test/lisp/emacs-lisp/eieio-tests/eieio-test-persist.el
index dfaa031844f..b87914c75e7 100644
--- a/test/lisp/emacs-lisp/eieio-tests/eieio-test-persist.el
+++ b/test/lisp/emacs-lisp/eieio-tests/eieio-test-persist.el
@@ -277,7 +277,7 @@ persistent class.")
     :type vector
     :initarg :random-vector)))
 
-(ert-deftest eieio-test-persist-hash-and-vector ()
+(defun eieio-test-persist-hash-and-vector ()
   (let* ((jane (make-instance 'person :name "Jane"))
          (bob  (make-instance 'person :name "Bob"))
          (hans (make-instance 'person :name "Hans"))
@@ -297,10 +297,18 @@ persistent class.")
     (aset (car (slot-value class 'janitors)) 1 hans)
     (aset (nth 1 (slot-value class 'janitors)) 1 dierdre)
     (unwind-protect
-        ;; FIXME: This should not error.
-        (should-error (persist-test-save-and-compare class))
+        (persist-test-save-and-compare class)
       (delete-file (oref class file)))))
 
+(ert-deftest eieio-persist-hash-and-vector-backward-compatibility ()
+  (let ((eieio-backward-compatibility t)) ; The default.
+    (eieio-test-persist-hash-and-vector)))
+
+(ert-deftest eieio-persist-hash-and-vector-no-backward-compatibility ()
+  :expected-result :failed ;; Bug#29220.
+  (let ((eieio-backward-compatibility nil))
+    (eieio-test-persist-hash-and-vector)))
+
 ;; Extra quotation of lists inside other objects (Gnus registry), also
 ;; bug#29220.
 
@@ -315,7 +323,7 @@ persistent class.")
     :initarg :htab
     :type hash-table)))
 
-(ert-deftest eieio-test-persist-interior-lists ()
+(defun eieio-test-persist-interior-lists ()
   (let* ((thing (make-instance
                  'eieio-container
                  :vec [nil]
@@ -335,8 +343,16 @@ persistent class.")
     (setf (nth 2 (cadar alst)) john
           (nth 2 (cadadr alst)) alexie)
     (unwind-protect
-        ;; FIXME: Should not error.
-        (should-error (persist-test-save-and-compare thing))
+        (persist-test-save-and-compare thing)
       (delete-file (slot-value thing 'file)))))
 
+(ert-deftest eieio-test-persist-interior-lists-backward-compatibility ()
+  (let ((eieio-backward-compatibility t)) ; The default.
+    (eieio-test-persist-interior-lists)))
+
+(ert-deftest eieio-test-persist-interior-lists-no-backward-compatibility ()
+  :expected-result :failed ;; Bug#29220.
+  (let ((eieio-backward-compatibility nil))
+    (eieio-test-persist-interior-lists)))
+
 ;;; eieio-test-persist.el ends here