Parser doesn't grok unqualified array element

Has this come up before?  6.4.2 and current sources both have
this problem:
select count(*) from pg_proc where pg_proc.proargtypes[0] = 701;
works, butselect count(*) from pg_proc where proargtypes[0] = 701;
fails with ERROR: Unable to locate type name 'proargtypes' in catalog

The grammar doesn't seem to have a case that allows for a subscripted
attribute name without a relation name in front of it.

It looks like fixing this might be as easy as making the "ColId"
cases in a_expr, b_expr, possibly other places include an
opt_indirection item like columnElem does.  But maybe there's
more to it than meets the eye?
        regards, tom lane