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

[Noam Postavsky <npostavs@users.sourceforge.net>]

On Thu, Jul 13, 2017 at 10:40 AM, Roland Winkler <winkler@gnu.org> wrote:
>
> The docstring of cl-progv says
>
>   Bind SYMBOLS to VALUES dynamically in BODY.
>
> But I am not sure whether this statement correctly reflects the actual
> code of cl-progv: cl-progv evaluates a let form at runtime, but it is
> not up to cl-progv to ensure dynamical binding.  I believe it depends on
> whether lexical binding is on or off whether the code currently used by
> cl-progv uses dynamical binding or lexical binding.

No, because cl-progv omits the second argument to `eval', which is the
same as passing nil. This guarantees the evaluated let-bindings are
dynamic bindings.

> So it is my ignorant guess that a consistent behavior of cl-progv with
> lexical binding on or off requires that the bindings of SYMBOLS to
> VALUES is also passed to `eval' as a second arg.

Doing this would mean that cl-progv would always bind lexically. We
could, in theory, change cl-progv to be that way, but it would be
backwards incompatible with previous Emacs versions, and with Common
Lisp.