bug#5973: Crash in get_next_display_element

[Eli Zaretskii]

> From: Juanma Barranquero <lekktu@gmail.com>
> Date: Tue, 20 Apr 2010 11:19:44 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>, david.reitter@gmail.com, 
> 5973@debbugs.gnu.org
> 
> I was debugging a simple defadvice to narrow-to-region. The "text" of
> the message is edebug trying to display END (the argument to n-t-r). I
> can reproduce the bug with just this advice:
> 
> (defadvice narrow-to-region (before heisenbug activate)
>   end)
> 
> I instrument it with C-u C-M-x

Instrument what? the defadvice, or something else?

> then C-h N to show NEWS, select a few lines and C-x n n.

Which lines did you select?  I think this is important, because
when a form is evaluated, Edebug displays in the echo area a message
showing the result of the evaluation, and if the result is a number,
it tries to display it as a character.  So the end of the region you
select determines what character will Edebug try to display.

> I get a message "Loading c:/emacs/lisp/international/uni-category.el
> (source)...done" and then the crash.

I don't see this message, perhaps because I used a different region.
I see all kinds of characters displayed, depending on what region I
select, but no crash.  Here's one example

      Result: 784 (#o1420, #x310, ?̐)

I'm guessing that the crash is somehow related to display of composite
characters (hint: try "C-u C-x =" on the weirdo question mark above),
which the bidi display does not handle correctly yet.