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

[Stefan Monnier]

>> It's messier than that: the issue is that it doesn't bind variables
>> which aren't used lexically (this is needed to avoid spurious warnings
>> about unused vars when the var is used in on place but not in another).
> BTW, a similar issue (pitfall) I faced was that I sometimes expect that
> when a variable is already bound, already the first appearance of the
> symbol would be turned into an equality test.

Hmm... so-called "non-linear patterns".  We should emit a warning when the
same var is used twice in a pattern, indeed, to avoid surprises.

We could also do as you describe but I'm not completely sure it's a good
idea (it begs questions about which equality test to use and when it
should be tested).

> I wonder if pcase variable bindings could be hygienic in the sense that
> they would use fresh symbols internally.  That would maybe make the
> semantics clearer.

I don't know what you mean by that.  Can you clarify?


        Stefan