casting between range types

At one point, the question of casting between range types came up. At
first, this seemed like a fairly reasonable suggestion, but now I don't
think I like the semantics.

A normal cast changes between essentially equivalent values in different
domains. For instance 3 as an int4 is equivalent to 3.0 as a numeric.

However, if we take the simple approach with range types and cast the
bounds, we end up with some weird situations.

First, a range is really a set. So if we take '[1,10)'::int4range and
cast that to numrange, we end up moving from a set of exactly 9 elements
to a set of an infinite number of elements. Going the other way is
probably worse.

Sometimes casts are a bit "lossy" and I suppose we could write that off.

But things get weirder when the total order is different (e.g. different
text collations). Then you end up with a completely different set of
values, which doesn't sound like a cast to me at all.

So, I'm leaning toward just not providing any casts from one range type
to another.

Thoughts?

RegardsJeff Davis