MDEV-8988: Apparently valid SQL query gives wrong result (nested WHERE)

- "Early NULLs filtering" optimization used to "peel off" Item_ref and
  Item_direct_ref wrappers from an outside column reference before
  adding "outer_table_col IS NOT NULL" into JOIN::outer_ref_cond.
- When this happened in a subquery that was evaluated in a post-GROUP-BY
  context, attempt to evaluate JOIN::outer_ref_cond would fetch an
  incorrect value of outer_table_col.

mysql-test/r/group_by.result

@@ -2640,3 +2640,41 @@ field1	field2
 DROP TABLE t1;
 DROP TABLE where_subselect;
 # End of Bug #58782
+#
+# MDEV-8988: Apparently valid SQL query gives wrong result (nested WHERE)
+#
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1 (a int, b int, c int);
+insert into t1 select A.a + 10*B.a, A.a, A.a + 10*B.a from t0 A, t0 B;
+insert into t1 values (NULL, NULL, NULL);
+create table t2 (c int, col1 int, key(c));
+insert into t2 select t1.a, 100000 from t1;
+analyze table t2;
+Table	Op	Msg_type	Msg_text
+test.t2	analyze	status	Table is already up to date
+explain 
+select 
+max(a)+ (select col1 from t2 where t2.c=t1.c)
+from t1 
+group by t1.b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	101	Using temporary; Using filesort
+2	DEPENDENT SUBQUERY	t2	ref	c	c	5	func	1	
+select 
+max(a) + (select col1 from t2 where t2.c=t1.c)
+from t1 
+group by t1.b;
+max(a) + (select col1 from t2 where t2.c=t1.c)
+NULL
+100090
+100091
+100092
+100093
+100094
+100095
+100096
+100097
+100098
+100099
+drop table t0,t1,t2;

mysql-test/t/group_by.test

@@ -1767,3 +1767,29 @@ DROP TABLE where_subselect;
 
 --echo # End of Bug #58782
 
+--echo #
+--echo # MDEV-8988: Apparently valid SQL query gives wrong result (nested WHERE)
+--echo #
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+create table t1 (a int, b int, c int);
+insert into t1 select A.a + 10*B.a, A.a, A.a + 10*B.a from t0 A, t0 B;
+insert into t1 values (NULL, NULL, NULL);
+
+create table t2 (c int, col1 int, key(c));
+insert into t2 select t1.a, 100000 from t1;
+analyze table t2;
+
+explain 
+select 
+  max(a)+ (select col1 from t2 where t2.c=t1.c)
+from t1 
+group by t1.b;
+
+select 
+  max(a) + (select col1 from t2 where t2.c=t1.c)
+from t1 
+group by t1.b;
+
+drop table t0,t1,t2;

sql/sql_select.cc

@@ -9359,7 +9359,7 @@ static void add_not_null_conds(JOIN *join)
           if (!referred_tab)
             continue;
           if (!(notnull= new (join->thd->mem_root)
-                Item_func_isnotnull(join->thd, not_null_item)))
+                Item_func_isnotnull(join->thd, item)))
             DBUG_VOID_RETURN;
           /*
             We need to do full fix_fields() call here in order to have correct