Subject: RE: bug#19424: 23.4; incorrect byte-compiler warning for `mode-line-format'

From: Drew Adams <[email-masked]>
Date: Sun, 21 Dec 2014 12:38:46 -0800 (PST)

> >     Warning: `default-mode-line-format' is an obsolete variable
> >     (as of Emacs 23.2); use `(default-value 'mode-line-format)'
> >     instead.
> 
> Why `default-value', but not `setq-default'?

Should be obvious: Because the intention of a given occurrence
of `default-mode-line-format' might not be to set the default
value of `mode-line-format'.

Consider code that used, for example:

(setq mode-line-format  default-mode-line-format)

A reasonable update of that code would likely be

(setq mode-line-format  (default-value 'mode-line-format))

That does not set the default value of `mode-line-format'.