bug#20268: 25.0.50; pcase-lambda broken

[Stefan Monnier]

> After the recent rewrite, pcase-lambda is broken. For example, eval the
> following to get 46422 instead of the correct value 65535.

>    (cl-some (pcase-lambda (`[fullsweep_after ,v]) v)
>             '([min_bin_vheap_size 46422]
>               [min_heap_size 233]
>               [fullsweep_after 65535]
>               [minor_gcs 40]))

Indeed, that's the semantics I chose.
The previous semantics was for the function to do nothing and return nil
if the arg doesn't match.  The new semantics is the same as the one used
by pcase-let.  It's not without its fault of course, but at least it does
correspond to the usual idea of "destructuring" and generates more
efficient code.

I think if you prefer to return nil, then the macro should look like

   (pcase-lambda ((`[fullsweep_after ,v]) v))

which would then naturally let you add additional cases like

   (pcase-lambda ((`[fullsweep_after ,v]) v)
                 ((`[min_heap_size ,v]) (/ v 2)))

Admittedly, for the current pcase-lambda (and pcase-let) macro, the
pcase.el code should be refined so as to emit a warning when it ends up
ignoring a constant like `fullsweep_after' above.


        Stefan