bug#62419: 28.2; Elisp let-bound buffer-local variable and kill-local-variable

[Eli Zaretskii <eliz@gnu.org>]

> From: Matthew Malcomson <hardenedapple@gmail.com>
> Date: Fri, 24 Mar 2023 13:37:57 +0000
> 
> I’m inlining some elisp which has behaviour I find unintuitive.
> To view the bug I would run each top-level form with C-x C-e in turn in an elisp buffer.
> This behaviour may be expected — the manual mentions something related — but I believe this is an unintended edge-case.
> N.b. the use of `auto-fill-function’ is just for a variable which turns buffer-local when set, not as anything related to this particular symbol.
> FWIW I believe this behaviour to be the root cause of https://github.com/joaotavora/yasnippet/issues/919 (which was closed due to not being able to reproduce it).
> 
> —————
> ;; Ensure that `auto-fill-function' has a buffer-local version and a global
> ;; version.
> (setq auto-fill-function 'local-symbol)
> (describe-variable 'auto-fill-function)
> ;; `auto-fill-function' is let-bound in the buffer scope
> (let ((auto-fill-function 'temp-symbol))
>   ;; Now there is no buffer-local variable for `auto-fill-function', but the
>   ;; `let' unwrapping info is still there.
>   (kill-local-variable 'auto-fill-function)
>   ;; Since the check in the emacs source is
>   ;; a) Is there a buffer-local variable.
>   ;; b) Is there a let-binding shadowing the current variable.
>   ;; Then this `setq' sets the *global* variable.
>   (setq auto-fill-function 'other-symbol))
> ;; Exiting the `let' has special handling to avoid resetting a local variable
> ;; when the local variable was `let' bound, which means that overall the `setq'
> ;; set the global variable and the `let' has been lost.
> (describe-variable 'auto-fill-function)
> —————
> 
> 
> I think the final state after having done the above should be:
> 1) Global value has not changed.
> 2) Local value has not changed.
> 
> While the observed state after the above is:
> 1) Global value has been set to `other-symbol'.
> 2) Local value has been removed.
> 
> - The `setq` inside the `let` sets the *global* value rather than creating a buffer-local variable.
> - The `let` on the buffer-local version of the variable is lost.

What is the purpose of doing this? what are you trying to accomplish,
and why?

> The manual for `make-variable-buffer-local` — in `(elisp) Creating Buffer-Local` — does mention that if a variable is in a `let` binding then a `setq` does not create a buffer-local version.
> That said, I’m guessing the intention of this behaviour is so a `let` binding on a global variable is modified rather than bypassed by a `setq`.
> I’d suggest that is not relevant to the above code since the use of `kill-local-variable` means the `let` is effectively lost already (e.g. it does not get “reset” on an unwind).

Did you also read about default-toplevel-value and
set-default-toplevel-value (in the "Default Value" node of the ELisp
manual)?

> I’m not proposing this as a change, just including it for extra context about the cause.
> I *believe* that the form of the C code around here looks like the special case of a forwarded variable not having a buffer-local value but having a buffer-local `let` binding could easily have been an oversight rather than intention.

Stefan, any comments?