Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> What's the alternative?  Transform malformed let in some undefined way?
>
> Yup.  Just like we've done so far.

I think I understand what you want.  This patch uses
macroexp--warn-and-return.  I'll try to move the checks in
bytecomp.el/cconv.el later.

Nathan