bug#23949: 25.0.95; Regression in handling error caused by (string-match-p "." nil)

[Eli Zaretskii]

> From: Kaushal Modi <kaushal.modi@gmail.com>
> Date: Tue, 12 Jul 2016 13:37:57 +0000
> Cc: 23949@debbugs.gnu.org
> 
> Some interesting discovery .. hope this helps debug this:
> 
> Searched the whole emacs source and my ~/.emacs.d for 'is relative, it is 
> com'. And the only place I found it
> was in lisp/progmodes/compile.el, line 2679:
> 
> =====
> km²~/downloads/:git/emacs> ag 'is relative, it is com' 
> lisp/progmodes/compile.el
> 2679:If DIRECTORY is relative, it is combined with `default-directory'.
> =====
> 
> Somehow "(nil" replaced "If DIRECT", the whole thing got lower-cased, string 
> after "com" in "combined" got
> truncated, and we got:
> 
> "(nilory is relative, it is com"

It's probably just some random garbage left on the stack.

Once again, the root cause lies in help-function-arglist or its
immediate callers.  Why does all this get called when Emacs signals an
error?