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Is there a way to tell if the current *.elc files were not compiled with the current Emacs?

I am trying to detect an error where emacs is updated, but old .elc files are left in the user elpa directory and they are not compatible.

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  • What do you mean by "the user elpa file"?
    – Drew
    Dec 30 '20 at 18:00
  • 1
    The file header of a .elc file tells you what Emacs build it was created with. Does that help?
    – Drew
    Dec 30 '20 at 18:02
  • @Drew I edited the question to indicate I mean in the user elpa directory, .e.g. where packages installed by the user usually go. Dec 30 '20 at 19:06
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Based on the hint from @Drew about the file header, this seems to work:

(defun elc-consistent-p (elc)
  "Return non-nil if ELC is consistent with current Emacs.
ELC is the path to a .elc file.
That means it was compiled with the same emacs major and minor
version as the current emacs."
  (string= (with-temp-buffer
         (insert-file-contents elc)
         (goto-char (point-min))
         (re-search-forward ";;; in Emacs version \\([0-9]\\{2\\}\\.[0-9]+\\)"
                nil t)
         (match-string 1))
       (format "%s.%s" emacs-major-version emacs-minor-version)))

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