bug#27674: 26.0.50; cl-progv: strange scoping due to implementation

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Michael Heerdegen <michael_heerdegen@web.de> writes:

> The way `cl-progv' is implemented, we have some strange effects
> happening to closures in the body.  For example, with lexical-binding
> on,
>
> (let ((x 0))
>   (cl-progv (list 'x) (list 1)
>     (funcall (lambda () x))))
>
> yields 0, and
>
> (cl-progv (list 'x) (list 1)
>   (funcall (lambda () x)))
>
> yields 1.  That isn't consistent (FWIW I would expect `1' in both
> cases).

IMO, this is a bug in your program, this yields 1:

(progn
  (defvar x)
  (let ((x 0))
    (cl-progv (list 'x) (list 1)
      (funcall (lambda () x)))))

Note that your second expression gives a compile warning (it's also
missing a defvar, though it happens to give the result you expect even
without that):

27674.el:10:25:Warning: reference to free variable ‘x’