bug#81602: 32.0.50; Is this a bug? setopt on a not-yet-loaded option breaks loading the file that defines it

Hello there!

I noticed some strange errors after trying to compile from master today.

I'm not sure whether this is a bug, although I suspect it might be, so I
thought I'd ask and share my findings.

My init.el sets (setopt eshell-prompt-function (lambda () ...)) at top
level.  Since commit 7e9e10d9842, the first M-x eshell of the session
gives me:

  Error (eshell): Unable to load Eshell module ‘eshell-prompt’

and an Eshell buffer with no prompt.  If I kill that buffer and run M-x
eshell again, everything works, which is what kept me looking in the
wrong place for a while.

Here is the same failure without my config:

  emacs -Q --batch --eval '(progn (setopt eshell-prompt-function (lambda () "$ ")) (require (quote em-prompt)))'
  => Wrong type argument: listp, #[nil ("$ ") (t)]

A symbol or t breaks the same way:

  emacs -Q --batch --eval '(progn (setopt erc-sasl-mechanism (quote plain)) (require (quote erc-sasl)))'
  => Wrong type argument: listp, plain

  emacs -Q --batch --eval '(progn (setopt vc-dir-auto-hide-up-to-date t) (require (quote vc-dir)))'
  => Wrong type argument: listp, t

What I think happens: when I call `setopt' before the defcustom has run,
`setopt--set' cannot type-check yet, so it stashes the value for later.

  ;; cus-edit.el
  (if (not (or type (get variable 'standard-value)))
	  ;; `custom-declare-variable' has not run yet.  Postpone the check.
	  (push value (get variable 'custom-check-values))
	...)

Later, `custom-declare-variable' picks the stash up again, and reads
each entry with `car':

  ;; custom.el
  (dolist (value (prog1 (nreverse (get symbol 'custom-check-values))
				   (put symbol 'custom-check-values nil)))
	(let ((type (get symbol 'custom-type)))
	  (when (and type
				 (not (widget-apply (widget-convert type)
									:match (car value))))
		(warn "Value previously set by setopt did not match %S's type %S:\n%S"
			  symbol type value))))

`setopt--set' pushes the bare value, `custom-declare-variable' reads it
as if someone had wrapped it in a list.  My lambda is not a list, so
`car' signals in the middle of evaluating the defcustom, and `load' of
em-prompt.el gives up right there.  A list value survives, though it
type-checks the list's car instead of the value I passed.

That `car' looks llike a leftover to me.  Before 7e9e10d9842,
`setopt--set' did (put variable 'custom-check-value (list value)) and
`custom-initialize-reset' read (car value), which matched.  The commit
renamed the property to `custom-check-values', switched it to a list of
bare values, moved the check into `custom-declare-variable', and kept
the `car'.  I found nothing in etc/NEWS about the storage format, so I
assume this was not on purpose.

Which side holds the contract?  If it is the wrapper, then cus-edit.el
needs one word:

  --- a/lisp/cus-edit.el
  +++ b/lisp/cus-edit.el
  @@ setopt--set
  -        (push value (get variable 'custom-check-values))
  +        (push (list value) (get variable 'custom-check-values))

If instead the stash should hold bare values, then I think custom.el is
the side to touch: maybe :match against VALUE, and print VALUE in the
warning rather than the wrapper.

A check that only wants to print a warning can currently abort the load
of the file it is checking.  Wrapping the dolist in
`with-demoted-errors' would keep a bad stashed value from taking the
file down with it.

Two details that made this hard to see from the Eshell side.
`eshell-load-modules' wraps the load in `condition-case-unless-debug',
so the wrong-type-argument never reaches me and I get the warning above
instead.  And `custom-declare-variable' clears `custom-check-values'
with `prog1' before the check signals, so nothing is pending on the
second try:

  (setopt eshell-prompt-function (lambda () "$ "))
  (get 'eshell-prompt-function 'custom-check-values)
  => (#[nil ("$ ") (t)])
  (require 'em-prompt)             ;; signals wrong-type-argument
  (get 'eshell-prompt-function 'custom-check-values)
  => nil
  (featurep 'em-prompt)
  => nil
  (require 'em-prompt)             ;; works, and the evidence is gone

Any option that has no `custom-type' yet when `setopt' runs looks like a
candidate, so the same config can carry several of these waiting to
fire, one per file.  I tried this to list the 'pending ones' in a
running Emacs:

  (let (found)
	(mapatoms (lambda (s)
				(dolist (v (get s 'custom-check-values))
				  (unless (listp v) (push s found)))))
	found)


Thanks,

-- Rahul

---


In GNU Emacs 32.0.50 (build 10, aarch64-apple-darwin25.5.0, NS
 appkit-2685.60 Version 26.5.2 (Build 25F84)) of 2026-08-11 built on
 MacBook-Pro.local
Repository revision: 902f8eb6334b49e08d7dd6ba6e4b4036175bd889
Repository branch: master
Windowing system distributor 'Apple', version 10.3.2685
System Description: macOS 26.5.2

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