bug#57397: cl-letf blindly macroexpands places

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> to support symbol macros better:
>
> #+begin_src emacs-lisp
> (defun cl--letf (bindings simplebinds binds body)
>   ;; It's not quite clear what the semantics of cl-letf should be...10..
>   (if (null bindings)
>       (if (and (null binds) (null simplebinds)) (macroexp-progn body)..20..)
>     (let* ((binding (car bindings))
>            (place (macroexpand (car binding) macroexpand-all-environment)));<--!!
>       (gv-letplace (getter setter) place..12..))))
> #+end_src
>
> I think this is an error in the general case.  It's at least always
> wrong when (car PLACE) is a macro name with a gv spec defined.

Indeed, defining a gv spec for a macro is fiddly.

> Maybe it's enough to expand only symbol macros?

Yes, that should still cover the original need in bug#26073 without
breaking your use case.

But regardless of this, we should probably turn `gv-synthetic-place`
into a function so it's more robust.


        Stefan