Commit fdb4f6635351b3f3b15b5e948c39e9efcdd615df causing Emacs to crash for me

Thu Aug 25 23:15:01 CEST 2011

On 2011-08-25 20:25 +0200, Štěpán Němec wrote:

> On Thu, 25 Aug 2011 19:12:20 +0200 Vegard Øye wrote:
>
>> Emacs signals the error "Variable binding depth exceeds
>> max-specpdl-size":
>>
>>     (setq x `#1=(a #1#))
>>
>> Then there's the case of self-referential lambda functions.
>> If you write the function below, how will it byte-compile?
>>
>>     '#1=(lambda () #1#)
>
> I don't think there is anything surprising about either of your
> examples: both ``' and `lambda' are macros, so if you create a
> cycle, it expands and expands and expands and expands and expands...
> you get the idea.

Ah, so /that's/ what's going on. Well, for what it's worth, the
lambda issue can be resolved then. This enters an infinite regress:

    (macroexpand-all '#1=(lambda () #1#))

But the following does not, since quoting the inner reference
prevents further expansion:

    (macroexpand-all '#1=(lambda () '#1#))

One can also avoid the backquote issue by constructing the list
in parts and using `nconc' to piece them together.

> OTOH, if I understand correctly, Tim said that his Emacs crashed.
> No matter what you do in Elisp, Emacs should never crash, so that
> would definitely be a candidate for `report-emacs-bug'.

Indeed. This should be sufficient to reproduce it:

    (eval-after-load 'comint
      '(define-key comint-mode-map "a" comint-mode-map))

In any case, I'll consider self-referential structures a big no-no
from now on.

-- 
Vegard