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

[Stefan Monnier <monnier@iro.umontreal.ca>]

> 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.


        Stefan