Cope with lateral references in the quals of a subquery RTE.

The qual pushdown logic assumed that all Vars in a restriction clause
must be Vars referencing subquery outputs; but since we introduced
LATERAL, it's possible for such a Var to be a lateral reference instead.
This led to an assertion failure in debug builds.  In a non-debug
build, there might be no ill effects (if qual_is_pushdown_safe decided
the qual was unsafe anyway), or we could get failures later due to
construction of an invalid plan.  I've not gone to much length to
characterize the possible failures, but at least segfaults in the
executor have been observed.

Given that this has been busted since 9.3 and it took this long for
anybody to notice, I judge that the case isn't worth going to great
lengths to optimize.  Hence, fix by just teaching qual_is_pushdown_safe
that such quals are unsafe to push down, matching the previous behavior
when it accidentally didn't fail.

Per report from Tom Ellis.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/20200713175124.GQ8220@cloudinit-builder

src/test/regress/expected/subselect.out

@@ -1159,6 +1159,59 @@ from int4_tbl;
  (4,5,6.0)
 (5 rows)
 
+--
+-- Check for sane handling of a lateral reference in a subquery's quals
+-- (most of the complication here is to prevent the test case from being
+-- flattened too much)
+--
+explain (verbose, costs off)
+select * from
+    int4_tbl i4,
+    lateral (
+        select i4.f1 > 1 as b, 1 as id
+        from (select random() order by 1) as t1
+      union all
+        select true as b, 2 as id
+    ) as t2
+where b and f1 >= 0;
+                 QUERY PLAN                 
+--------------------------------------------
+ Nested Loop
+   Output: i4.f1, ((i4.f1 > 1)), (1)
+   ->  Seq Scan on public.int4_tbl i4
+         Output: i4.f1
+         Filter: (i4.f1 >= 0)
+   ->  Append
+         ->  Subquery Scan on t1
+               Output: (i4.f1 > 1), 1
+               Filter: (i4.f1 > 1)
+               ->  Sort
+                     Output: (random())
+                     Sort Key: (random())
+                     ->  Result
+                           Output: random()
+         ->  Result
+               Output: true, 2
+(16 rows)
+
+select * from
+    int4_tbl i4,
+    lateral (
+        select i4.f1 > 1 as b, 1 as id
+        from (select random() order by 1) as t1
+      union all
+        select true as b, 2 as id
+    ) as t2
+where b and f1 >= 0;
+     f1     | b | id 
+------------+---+----
+          0 | t |  2
+     123456 | t |  1
+     123456 | t |  2
+ 2147483647 | t |  1
+ 2147483647 | t |  2
+(5 rows)
+
 --
 -- Check that volatile quals aren't pushed down past a DISTINCT:
 -- nextval() should not be called more than the nominal number of times

src/test/regress/sql/subselect.sql

@@ -627,6 +627,32 @@ select (select q from
          ) q )
 from int4_tbl;
 
+--
+-- Check for sane handling of a lateral reference in a subquery's quals
+-- (most of the complication here is to prevent the test case from being
+-- flattened too much)
+--
+explain (verbose, costs off)
+select * from
+    int4_tbl i4,
+    lateral (
+        select i4.f1 > 1 as b, 1 as id
+        from (select random() order by 1) as t1
+      union all
+        select true as b, 2 as id
+    ) as t2
+where b and f1 >= 0;
+
+select * from
+    int4_tbl i4,
+    lateral (
+        select i4.f1 > 1 as b, 1 as id
+        from (select random() order by 1) as t1
+      union all
+        select true as b, 2 as id
+    ) as t2
+where b and f1 >= 0;
+
 --
 -- Check that volatile quals aren't pushed down past a DISTINCT:
 -- nextval() should not be called more than the nominal number of times