Re: Cross-type index comparison support in contrib/btree_gin

I wrote:
> I forgot to mention a couple of questions for review:

> Should we adjust the documentation of comparePartial() to promise
> explicitly that partial_key is the same datum returned by extractQuery?
> By my reading, it kind of implies that, but it's not quite black and
> white.

> In the 0005 patch, I relied on date2timestamp_opt_overflow and
> its siblings where available. But some of the conversions such
> as timestamptz-to-timestamp don't have one of those, so I was
> forced to copy-and-paste some fairly low-level code. Would it
> make sense to refactor the related core routines to expose
> xxx2yyy_opt_overflow interfaces, extending what 5bc450629 and
> 52ad1e659 did?

After further review it seems like both of those things would be
improvements, so here's a v2 that does it like that. This also
adds a PG_USED_FOR_ASSERTS_ONLY marker whose lack was pointed
out by the cfbot; no other meaningful changes.

regards, tom lane