bug#68509: 30.0.50; pcase-dolist matches backquote pattern incorrectly

[Stefan Monnier]

> What about adding extra option/keyword and also showing a runtime
> warning when pcase-dolist/pcase-let encounter something that does not
> match the pattern?

In the current use of those primitives, it's routine/normal/common for
a pattern like `(,a ,b) to encounter lists of different length than 2,
so a warning should be emitted only if specifically requested via an
option/keyword.

I think it would be OK to emit a warning (even if not specifically
requested via an option/keyword) when a value like (help . me) fails to
match `(a . ,b), but as mentioned I don't know a good way to distinguish
those two kinds of cases.


        Stefan