bug#26624: 26.0.50; Generalized variable `buffer-local-value' does't restore local flag

[Philipp Stephani <p.stephani2@gmail.com>]

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Philipp Stephani <p.stephani2@gmail.com> schrieb am So., 23. Apr. 2017 um
19:14 Uhr:

>
> In *scratch*, evaluate:
>
> (defvar foo-test-var nil)
> (with-temp-buffer
>   (list (list (buffer-local-value 'foo-test-var (current-buffer))
>               (local-variable-p 'foo-test-var)
>               (local-variable-if-set-p 'foo-test-var))
>         (cl-letf (((buffer-local-value 'foo-test-var (current-buffer))
> 123))
>           (list (buffer-local-value 'foo-test-var (current-buffer))
>                 (local-variable-p 'foo-test-var)
>                 (local-variable-if-set-p 'foo-test-var)))
>         (list (buffer-local-value 'foo-test-var (current-buffer))
>               (local-variable-p 'foo-test-var)
>               (local-variable-if-set-p 'foo-test-var))))
>
> The result is:
>
> ((nil nil nil) (123 t t) (nil t t))
>
> But expected is:
>
> ((nil nil nil) (123 t t) (nil nil nil))
>
> i.e. the local flag of the variable should be reset.
>
>
It's possible to fix this (see attached patch), but at the expense of
breaking other valid use cases such as (cl-incf (buffer-local-value ...)).
Not sure whether the bug can be fixed at all without breaking other stuff.

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From 3789b7b843ec417ae3be5d0f24409559a46bcc93 Mon Sep 17 00:00:00 2001
From: Philipp Stephani <phst@google.com>
Date: Fri, 16 Jun 2017 22:55:52 +0200
Subject: [PATCH 1/2] Have `cl-letf' restore buffer-local status (Bug#26624)

* lisp/emacs-lisp/gv.el (buffer-local-value): Record and restore
whether the variable was buffer-local; used for `cl-letf'.
* test/lisp/emacs-lisp/gv-tests.el (gv-tests--bug26624): Add unit
test.
---
 lisp/emacs-lisp/gv.el            | 15 +++++++++---
 test/lisp/emacs-lisp/gv-tests.el | 51 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+), 3 deletions(-)
 create mode 100644 test/lisp/emacs-lisp/gv-tests.el

diff --git a/lisp/emacs-lisp/gv.el b/lisp/emacs-lisp/gv.el
index c5c12a6414..a614d675af 100644
--- a/lisp/emacs-lisp/gv.el
+++ b/lisp/emacs-lisp/gv.el
@@ -372,9 +372,18 @@ setf
 (gv-define-setter window-point (v &optional w) `(set-window-point ,w ,v))
 (gv-define-setter window-start (v &optional w) `(set-window-start ,w ,v))
 
-(gv-define-setter buffer-local-value (val var buf)
-  (macroexp-let2 nil v val
-    `(with-current-buffer ,buf (set (make-local-variable ,var) ,v))))
+(gv-define-expander buffer-local-value
+  (lambda (do var buf)
+    (macroexp-let2* nil ((var var) (buf buf))
+      (funcall do
+               `(if (local-variable-p ,var ,buf)
+                    (buffer-local-value ,var ,buf)
+                  #1='#:unbound)
+               (lambda (val)
+                 `(with-current-buffer ,buf
+                    (if (eq ,val #1#)
+                        (kill-local-variable ,var)
+                      (set (make-local-variable ,var) ,val))))))))
 
 (gv-define-expander alist-get
   (lambda (do key alist &optional default remove)
diff --git a/test/lisp/emacs-lisp/gv-tests.el b/test/lisp/emacs-lisp/gv-tests.el
new file mode 100644
index 0000000000..b49c12ddf2
--- /dev/null
+++ b/test/lisp/emacs-lisp/gv-tests.el
@@ -0,0 +1,51 @@
+;;; gv-tests.el --- unit tests for gv.el             -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2017 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Unit tests for lisp/emacs-lisp/gv.el.
+
+;;; Code:
+
+(defvar gv-tests--var 'default "Test variable.")
+
+(ert-deftest gv-tests--bug26624 ()
+  "Checks that Bug#26624 is fixed."
+  (with-temp-buffer
+    (let ((var-calls 0) (buf-calls 0))
+      (should (equal (buffer-local-value 'gv-tests--var (current-buffer))
+                     'default))
+      (should-not (local-variable-p 'gv-tests--var))
+      (should-not (local-variable-if-set-p 'gv-tests--var))
+      (cl-letf (((buffer-local-value
+                  (progn (cl-incf var-calls) 'gv-tests--var)
+                  (progn (cl-incf buf-calls) (current-buffer)))
+                 'unbound))
+        (should (equal (buffer-local-value 'gv-tests--var (current-buffer))
+                       'unbound))
+        (should (local-variable-p 'gv-tests--var))
+        (should (local-variable-if-set-p 'gv-tests--var)))
+      (should (equal (buffer-local-value 'gv-tests--var (current-buffer))
+                     'default))
+      (should-not (local-variable-p 'gv-tests--var))
+      (should-not (local-variable-if-set-p 'gv-tests--var))
+      (should (equal var-calls 1))
+      (should (equal buf-calls 1)))))
+
+;;; gv-tests.el ends here
-- 
2.13.1