bug#65209: 30.0.50; Unexpected behaviour of setq-local

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Evaluate the following

(progn
   (defvar my-var :default-value)
   (make-variable-buffer-local 'my-var)
   (let ((my-var :let-value))
     (setq-local my-var :buffer-local-new))
   my-var)
=> :default-value

(progn
   (defvar my-var1 :default-value)
   (make-variable-buffer-local 'my-var1)
   (setq my-var1 :buffer-local)
   (let ((my-var1 :let-value))
     (setq-local my-var1 :buffer-local-new))
   my-var1)
=> :buffer-local

In both cases, setq-local has no effect.

(I stumbled over this because lexical-binding is set to nil in *scratch*
when using persistent-scratch (MELPA).  It's quite convolutetd, but in
short: Custom loads persistent-scratch, which loads the contents of
*scratch*, and calls lisp-interaction-mode.  Lisp-interaction-mode
setq-local's lexical-binding to t, but this doesn't work because all
this happens during an eval-buffer that has a specbind of
lexical-binding to nil.)

In GNU Emacs 30.0.50 (build 1, aarch64-apple-darwin22.5.0, NS
  appkit-2299.60 Version 13.4.1 (Build 22F82)) of 2023-07-13 built on
  Mini.fritz.box
Repository revision: e0244f58042740c8fe914e7abe5b02611b176bc2
Repository branch: master
Windowing system distributor 'Apple', version 10.3.2299
System Description:  macOS 13.5