bug#81493: 30.2; Incorrect byte-compiler warning about unused value

Michael Heerdegen via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <[email protected]> Wed, 29 Jul 2026 06:23:25 +0200

Newsgroups	gmane.emacs.bugs
Message-ID	<[email protected]>

"Drew Adams" <[email protected]> writes:

> I get this byte-compiler warning, which is incorrect, AFAICT: 
>
>   value from call to 'format' is unused

Reproducer:

#+begin_src emacs-lisp
(defun drew-test ()
  (when (format "%d" 5)
    (message "%s" (format "%d" 5))))
#+end_src

Your macro (not function, macro: `diredp-mark-if') call expands to
something similar.  I haven't checked the details but I guess that the
warning is intentional (the return value of `format' is always non-nil
so the actual return value is really not used).  I would rather fix the
macro and move the `when' test out of the expansion.

Note the compiler generates warnings for the expanded code - and above
the warning is (only) about the first `format' call.  You can interpret
it as a warning about a partially poorly written macro.

Michael.