bug#45619: 28.0.50; pcase-let on MacOS doesn't work

[Stefan Monnier]

> (pcase-let ((default-directory "/tmp/"))
>   (call-interactively 'find-file))
>
> I wonder what is messier: the warnings or the semantics resulting from
> avoiding the warnings.  I guess people would more often complain about
> the warnings...

I think the cleaner semantics is to say that if variables bound by pcase
are dynamically scoped the behavior is "undefined", and to add
a check&warning about it in pcase (the problem with the old "unused
var" warning is that it was cumbersome to avoid it, whereas this new
warning would be easy to avoid by moving the binding to a separate
`let`).


        Stefan