Subject: 28.2; provided-mode-derived-p doesn't work when passed an alias in MODES

From: Damien Cassou <[email-masked]>
Date: Sun, 07 May 2023 11:49:37 +0200

Hi,

The function `provided-mode-derived-p` (used by `derived-mode-p`) has
the code below. The first 3 lines of the function takes care of
converting the first argument MODE if it is an alias (as
`javascript-mode` is an alias for `js-mode`). This works great when
calling the function this way:

  (provided-mode-derived-p 'javascript-mode 'prog-mode)
    ⇒ prog-mode

But if the second argument MODES contains an alias, the code has no
conversion mechanism and the line below returns nil even though I
expect it to return non-nil:

  (provided-mode-derived-p 'javacript-mode 'javascript-mode)
    ⇒ nil

Is that a bug?

(defun provided-mode-derived-p (mode &rest modes)
  "Non-nil if MODE is derived from one of MODES.
Uses the `derived-mode-parent' property of the symbol to trace backwards.
If you just want to check `major-mode', use `derived-mode-p'."
  ;; If MODE is an alias, then look up the real mode function first.
  (when-let ((alias (symbol-function mode)))
    (when (symbolp alias)
      (setq mode alias)))
  (while
      (and
       (not (memq mode modes))
       (let* ((parent (get mode 'derived-mode-parent))
              (parentfn (symbol-function parent)))
         (setq mode (if (and parentfn (symbolp parentfn)) parentfn parent)))))
  mode)

-- 
Damien Cassou

"Success is the ability to go from one failure to another without
losing enthusiasm." --Winston Churchill