Re: 31395511: "Don’t attempt to modify constant strings"

[Dmitry Gutov <dgutov@yandex.ru>]

On 05.06.2020 18:25, João Távora wrote:
> But if you evaluate it, that doesn't happen, which is probably even
> worse.
> 
> And this is even stranger, IMO:
> 
>      (defun example-bug2 ()
>        (eq (make-text-button
>                       "example" nil
>                       'action (lambda (_) (message "action 1")))
>            (make-text-button
>                       "example" nil
>                       'action (lambda (_) (message "action 2")))))
>       
>      (defun example-bug3 ()
>        (eq "example" "example"))
>       
>      (defun example-bug4 ()
>        (let ((str1 "example")
>              (str2 "example"))
>          (eq str1 str2)))
>       
>      (list (example-bug2) (example-bug3) (example-bug4))
> 
>    when compiled, last form returns (t nil t)   in emacs 27,
>    when compiled, last form returns (nil nil t) in emacs 28.
>    when evaluated, last form returns (nil nil nil) in emacs 28.
> 
> For comparison, example-bug4 is valid Common Lisp and will return nil in
> every Common Lisp implementation I know (I tested with ACL and SBCL),
> regardless of whether compiled or evaluated.  I'm reasonably confident
> there's somewhere in the Hyperspec where that behaviour may be specified
> (I trust some CL pope will find it for me;-)  )

FWIW, it's a non-intuitive limitation for me as well.

But you can give bug#40671 a read, so see some context you might be missing.