bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces

[Okamsn via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Hello,

Currently, the use

     (let ((arr (vector 0 1 2 3 4 5 6)))
       (setf (map-elt (cl-subseq arr 3) 0)
             27)
       arr)

expands to

     (let ((arr (vector 0 1 2 3 4 5 6)))
       (let* ((v arr))
         (condition-case nil
             (with-no-warnings
               (map-put! (cl-subseq v 3) 0 27 nil))
           (map-not-inplace
            (let* ((new (map-insert (cl-subseq v 3) 0 27)))
              (progn
                (cl-replace v new :start1 3 :end1 nil)
                new))
            27)))
       arr)

which does not modify the original variable `arr` due to how `map-put!` 
is being used. With the attached patch, it would expand to

     (let ((arr (vector 0 1 2 3 4 5 6)))
       (let* ((v arr))
         (condition-case nil
             (with-no-warnings
               (let* ((m (cl-subseq v 3)))
                 (progn
                   (map-put! m 0 27 nil)
                   (let* ((new m))
                     (progn
                       (cl-replace v new :start1 3 :end1 nil)
                       new))
                   27)))
           (map-not-inplace
            (let* ((new (map-insert (cl-subseq v 3) 0 27)))
              (progn
                (cl-replace v new :start1 3 :end1 nil)
                new))
            27)))
       arr)

which correctly sets the value using `cl-replace` as the setter for 
`cl-subseq`.

Thank you.

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From 1ac65858dc2f241975df8620ac79c973fcc6dad4 Mon Sep 17 00:00:00 2001
From: Earl Hyatt <okamsn@protonmail.com>
Date: Mon, 22 Apr 2024 21:45:03 -0400
Subject: [PATCH] Make setf with map-elt work with subplaces.

* lisp/emacs-lisp/map.el (map-elt): Using the setting function defined
by 'gv-letplace' to make sure that we capture the modification, even
when using 'map-put!'.  Otherwise, the sub-place might be understood as
a normal function call that will create the value to modify instead.

* test/lisp/emacs-lisp/map-tests.el (test-setf-map-with-function): Test
that sub-places work with 'map-elt' as expected.  For example, the use
of 'substring' should be understood as wanting to modify an existing
string, not wanting to modify the new string that would be created by a
call to 'substring'.
---
 lisp/emacs-lisp/map.el            | 6 +++++-
 test/lisp/emacs-lisp/map-tests.el | 5 +++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/lisp/emacs-lisp/map.el b/lisp/emacs-lisp/map.el
index d3d71a36ee4..facfdd8de7b 100644
--- a/lisp/emacs-lisp/map.el
+++ b/lisp/emacs-lisp/map.el
@@ -167,7 +167,11 @@ map-elt
                        `(condition-case nil
                             ;; Silence warnings about the hidden 4th arg.
                             (with-no-warnings
-                              (map-put! ,mgetter ,key ,v ,testfn))
+                              ,(macroexp-let2 nil m mgetter
+                                 `(progn
+                                    (map-put! ,m ,key ,v ,testfn)
+                                    ,(funcall msetter m)
+                                    ,v)))
                           (map-not-inplace
                            ,(funcall msetter
                                      `(map-insert ,mgetter ,key ,v))
diff --git a/test/lisp/emacs-lisp/map-tests.el b/test/lisp/emacs-lisp/map-tests.el
index dc8121b0582..c79bac54cb3 100644
--- a/test/lisp/emacs-lisp/map-tests.el
+++ b/test/lisp/emacs-lisp/map-tests.el
@@ -723,6 +723,11 @@ test-setf-map-with-function
     ;; Check that the function is only called once.
     (should (= num 1))))
 
+(ert-deftest test-setf-map-with-subplace ()
+  (let ((arr (string ?0 ?1 ?2 ?3 ?4 ?5 ?6)))
+    (setf (map-elt (substring arr 3) 0) ?x)
+    (should (equal arr (string ?0 ?1 ?2 ?x ?4 ?5 ?6)))))
+
 (ert-deftest test-map-plist-member ()
   "Test `map--plist-member' and `map--plist-member-1'."
   (dolist (mem '(map--plist-member map--plist-member-1))
-- 
2.34.1