bug#3467: 23.0.94; let + make-local-variable => let value made global

[Lennart Borgman]

On Fri, Jun 5, 2009 at 4:36 PM, Stefan Monnier<monnier@iro.umontreal.ca> wrote:
>> If you eval these lines
>>   (defvar w14 "global")
>>   (defvar w15 "global")
>>   (let ((w14 "let")
>>         (w15 "let"))
>>     (set (make-local-variable 'w14) "local")
>>     (message "w14 maybe let: in buffer=%S, global=%S" w14 (default-value 
>> 'w14))
>>     (message "w15 maybe let: in buffer=%S, global=%S" w15 (default-value 
>> 'w15)))
>>   (message "w14 top level: in buffer=%S, global=%S" w14 (default-value 'w14))
>>   (message "w15 top level: in buffer=%S, global=%S" w15 (default-value 'w15))
>
>> the output will be
>
>>   w14 maybe let: in buffer="local", global="let"
>>   w15 maybe let: in buffer="let", global="let"
>>   w14 top level: in buffer="global", global="let"
>>   w15 top level: in buffer="global", global="global"
>
>> All values here except w14 global value on next last line are arguably
>> correct. The last value of w14 should be "global", not "let".
>
>> It looks like perhaps the call to (make-local-variable w14) does not
>> mark the "global let" value of w14 as let bound (or removes that
>> mark).
>
> Given the way let-binding and buffer-local bindings are currently
> implemented, it's difficult to make it work correctly in all corner
> cases, and even more so without slowing down the common case.
> So don't hold your breath.

I gave a suggestion in the next message for how to implement this:
Check buffer and frame localness before unbind_to in Flet. Would that
really be expensive?