Re-allow duplicate aliases within aliased JOINs.

Although the SQL spec forbids duplicate table aliases, historically
we've allowed queries like
    SELECT ... FROM tab1 x CROSS JOIN (tab2 x CROSS JOIN tab3 y) z
on the grounds that the aliased join (z) hides the aliases within it,
therefore there is no conflict between the two RTEs named "x".  The
LATERAL patch broke this, on the misguided basis that "x" could be
ambiguous if tab3 were a LATERAL subquery.  To avoid breaking existing
queries, it's better to allow this situation and complain only if
tab3 actually does contain an ambiguous reference.  We need only remove
the check that was throwing an error, because the column lookup code
is already prepared to handle ambiguous references.  Per bug #8444.

src/test/regress/sql/join.sql

@@ -890,6 +890,15 @@ SELECT * FROM
 
 rollback;
 
+-- bug #8444: we've historically allowed duplicate aliases within aliased JOINs
+
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y) j on q1 = f1; -- error
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y) j on q1 = y.f1; -- error
+select * from
+  int8_tbl x join (int4_tbl x cross join int4_tbl y(ff)) j on q1 = f1; -- ok
+
 --
 -- Test LATERAL
 --
@@ -1077,5 +1086,8 @@ select f1,g from int4_tbl a cross join (select a.f1 as g) ss;
 -- SQL:2008 says the left table is in scope but illegal to access here
 select f1,g from int4_tbl a right join lateral generate_series(0, a.f1) g on true;
 select f1,g from int4_tbl a full join lateral generate_series(0, a.f1) g on true;
+-- check we complain about ambiguous table references
+select * from
+  int8_tbl x cross join (int4_tbl x cross join lateral (select x.f1) ss);
 -- LATERAL can be used to put an aggregate into the FROM clause of its query
 select 1 from tenk1 a, lateral (select max(a.unique1) from int4_tbl b) ss;