Subject: Re: bug#15103: 24.3.50; Wrong byte compiler warning

From: Stefan Monnier <[email-masked]>
Date: Thu, 15 Aug 2013 10:50:25 -0400

> I have a file test.el:
> (condition-case err
>     (ignore)
>   (file-error (ignore))
>   (error (message "%s" (error-message-string err))))

> When I compile it with --eval '(setq byte-compile-force-lexical-warnings t)'
> I get the byte compiler warning

> In toplevel form:
> test.el:1:1:Warning: Unused lexical argument `err'
> Wrote /home/albinus/tmp/test.elc

> Note, that this warning does not appear when the first condition-case
> handler does not exist.

Yup, same kind of problem as discussed in the "Cleaning up code" thread.
When compiling for lexical scoping, the condition case above gets turned
internally into something akin to:

 (condition-case :use-lexical-calling-convention
     (ignore)
   (file-error (lambda (err) (ignore)))
   (error (lambda (err) (message "%s" (error-message-string err)))))

so as you can see, we now have two separate declarations of `err' one of
which is unused.
There's a secondary bug as well in that the warning says "argument"
rather than "variable", but that's minor.

        Stefan