bug#66938: 30.0.50 [PATCH]: Make EIEIO :accessor behave like :reader when reading a slot's value

[Brandon Irizarry <brandon.irizarry@gmail.com>]

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Hope everyone is well.

I've included a suggested patch for 'eieio.el', which slightly
modifies the behavior of EIEIO's 'defclass'. When playing around with
'eieio-instance-inheritor' as a base class, I noticed that calls to
the accessor (when used as a getter) weren't delegating to the parent
instance (as defined when calling 'clone'). That is, an object clone that
doesn't set
its fields directly will report 'nil' when the accessor method is used as a
getter for some slot.

However, neither 'oref' nor ':reader' methods share this problem.

The included patch duplicates the code for the reader method defined for
the ':reader' case. In particular, it removes the 'slot-unboundp' check,
which appeared
to be the real culprit here. I've also deleted the FIXME comment which
asked, "Why is this
different from the :reader case?" since it no longer differs from it.

There is some more context provided (along with an example) in the
commit message included in the patch, in case it helps.

I know this isn't earth-shattering, but I couldn't help but notice the
issue;
and, at least to me, it seemed like incorrect behavior. (Otherwise, in
order to
obtain the delegation behavior for a given slot, I'd be forced to define
 ':reader' and ':writer' separately, which feels rather verbose, given we
have ':accessor'
for this exact purpose.)

- Brandon

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From 2d40d63d8738dc44b02ca61843fa61956958e84e Mon Sep 17 00:00:00 2001
From: Brandon <brandon.irizarry@gmail.com>
Date: Sat, 4 Nov 2023 17:11:32 -0400
Subject: [PATCH] Make EIEIO ':accessor' behave like ':reader' when reading

* lisp/emacs-lisp/eieio.el (defclass): Remove 'slot-boundp' check for
:accessor's getter

Clones of instances of subclasses of 'eieio-instance-inheritor' don't
delegate to their ':parent-instance' field when reading object fields
using ':accessor'.

Say I have this code:

(defclass foo (eieio-instance-inheritor)
  ((x :initarg :x
      :accessor ref-x
      :reader get-x)))

(setq obj1 (foo :x 4))   ; #s(foo eieio--unbound 4)
(setq obj2 (clone obj1)) ; #s(foo #s(foo eieio--unbound 4) eieio--unbound)

(ref-x obj1) ; 4, which is correct.
(ref-x obj2) ; nil. This is what we want to fix.
(get-x obj2) ; Gives us 4: access via the reader performs delegation.

My impression is that ':accessor' should behave as if ':reader' and
':writer' had been provided separately.

With this patch, '(ref-x obj2)' now uses the exact same method as
':reader', and so would give us 4, as desired.
---
 lisp/emacs-lisp/eieio.el | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/lisp/emacs-lisp/eieio.el b/lisp/emacs-lisp/eieio.el
index 39a5fd5b19c..8224606ec57 100644
--- a/lisp/emacs-lisp/eieio.el
+++ b/lisp/emacs-lisp/eieio.el
@@ -213,9 +213,8 @@ defclass
                    ,(internal--format-docstring-line
                      "Retrieve the slot `%S' from an object of class `%S'."
                      sname name)
-                   ;; FIXME: Why is this different from the :reader case?
-                   (if (slot-boundp this ',sname) (eieio-oref this ',sname)))
-                accessors)
+                   (slot-value this ',sname))
+                  accessors)
           (when (and eieio-backward-compatibility (eq alloc :class))
             ;; FIXME: How could I declare this *method* as obsolete.
             (push `(cl-defmethod ,acces ((this (subclass ,name)))
-- 
2.39.2