Re: CREATE CAST allows creation of binary-coercible cast to range over domain

I wrote:
> I think it's an oversight that we allow binary-coercible casts
> involving range types at all. There are no other nesting cases to
> worry about, because CreateCast already rejects binary-coercible casts
> applied to composites and arrays, explaining
> * We know that composite, enum and array types are never binary-
> * compatible with each other. They all have OIDs embedded in them.
> But range types also embed their own OID, so I don't see why that
> concern doesn't apply to them.

In short, more or less as attached. (I didn't bother with a
regression test, since none of the adjacent error checks are
covered either.)

I'm unsure whether to back-patch this. If somebody were using
such a cast today, it might seem to mostly work, as long as the
two kinds of ranges had the same behavior. So they might not
appreciate us breaking it in a minor release. Maybe we should
put this into 17 but not further back.

regards, tom lane