bug#77627: 31.0.50; prettify-symbols-mode fails when replacement CHARACTER is a composition rule

[Visuwesh]

[செவ்வாய் ஏப்ரல் 08, 2025] Visuwesh wrote:

> As per the docstring of prettify-symbols-alist, the replacement for any
> SYMBOL can be:
>
>     Each element looks like (SYMBOL . CHARACTER), where the symbol
>     matching SYMBOL (a string, not a regexp) will be shown as
>     CHARACTER instead.
>
> where CHARACTER being list or vector implies that it is a composition
> rule.  However, prettify-symbols--composition-displayable-p fails
> because it assumes CHARACTER is always a character (i.e., integer).
>
> To reproduce,
>
>     1. emacs -Q
>     2. M-: (setq prettify-symbols-alist '(("\\left\\{" . (?⎨ (tc . Bc) ?⎧ (Bc 
> . tc) ?⎩)))) RET
>     3. M-x toggle-debug-on-error RET
>     4. M-x prettify-symbols-mode RET
>     5. Witness the backtrace.

OK, the new predicate does consider composition rule but there's a typo:

diff --git a/lisp/progmodes/prog-mode.el b/lisp/progmodes/prog-mode.el
index 33e0d354fee..6c6b14d455a 100644
--- a/lisp/progmodes/prog-mode.el
+++ b/lisp/progmodes/prog-mode.el
@@ -244,8 +244,8 @@ prettify-symbols--composition-displayable-p
     ;; check that every even-indexed element is displayable
     (seq-every-p
      (lambda (idx-elt)
-       (if (evenp (car idx-elt))
-           (char-displayable-on-frame-p (cdr idx-elt))
+       (if (evenp (cdr idx-elt))
+           (char-displayable-on-frame-p (car idx-elt))
          t))
      (seq-map-indexed #'cons composition)))
    (t