Better error message for a small problem with WITH RECURSIVE

Quick, what's wrong with this query?

regression=# with q(x) as (select 1 union all select x+1 from q where x<10)
regression-# select * from q;
ERROR: relation "q" does not exist
LINE 1: with q(x) as (select 1 union all select x+1 from q where x<1...
^

The problem is that I forgot to say RECURSIVE. But the error message
is certainly pretty unhelpful. The code is following the SQL spec rule,
which says that for a non-recursive WITH query, the query's name isn't
in scope until after you've parsed it. So you get "does not exist"
rather than something that would clue you in.

I've made this same mistake at least once a day for the past week,
and taken an unreasonable amount of time to figure it out each time :-(
So I think we need a better error message here.

We can't just monkey with the scope rules, because that could change
the meaning of queries that *are* valid, eg a query name could be a
reference to some outer-level WITH. (Of course you'd have to be pretty
nuts to use the same query name at multiple levels of a single SELECT,
and even more nuts to arrange things so that the intended reference is
not the most closely nested one, but spec is spec.) What we can do is
keep a list of "not yet parsed WITH-names" in ParseState, and check
through that list when about to fail for relation-not-found, and issue
a suitable message hinting that maybe you forgot RECURSIVE if we find
a match.

I would think this is overkill, except I've made the same darn mistake
one time too many. It seems clear to me that a lot of other people will
make it too, and if the error message isn't more helpful a lot of time
will get wasted. Barring loud objections, I'm gonna go change it.

regards, tom lane