Item_func_isnotnull::not_null_tables - return 0 if not top-level item

mysql-test/r/join_outer.result:
  test for some obscure usage of IS NOT NULL
mysql-test/t/join_outer.test:
  test for some obscure usage of IS NOT NULL

mysql-test/r/join_outer.result

@@ -650,6 +650,9 @@ select * from t1 natural left join t2 natural left join t3;
 i	i	i
 1	NULL	NULL
 2	2	2
+select * from t1 natural left join t2 where (t2.i is not null)=0;
+i	i
+1	NULL
 drop table t1,t2,t3;
 create table t1 (f1 integer,f2 integer,f3 integer);
 create table t2 (f2 integer,f4 integer);

mysql-test/t/join_outer.test

@@ -430,6 +430,7 @@ insert into t1 values(1),(2);
 insert into t2 values(2),(3);
 insert into t3 values(2),(4);
 select * from t1 natural left join t2 natural left join t3;
+select * from t1 natural left join t2 where (t2.i is not null)=0;
 drop table t1,t2,t3;
 
 #

sql/item_cmpfunc.h

@@ -905,8 +905,9 @@ public:
 
 class Item_func_isnotnull :public Item_bool_func
 {
+  bool abort_on_null;
 public:
-  Item_func_isnotnull(Item *a) :Item_bool_func(a) {}
+  Item_func_isnotnull(Item *a) :Item_bool_func(a), abort_on_null(0) {}
   longlong val_int();
   enum Functype functype() const { return ISNOTNULL_FUNC; }
   void fix_length_and_dec()
@@ -915,10 +916,11 @@ public:
   }
   const char *func_name() const { return "isnotnull"; }
   optimize_type select_optimize() const { return OPTIMIZE_NULL; }
-  table_map not_null_tables() const { return used_tables(); }
+  table_map not_null_tables() const { return abort_on_null ? used_tables() : 0; }
   Item *neg_transformer(THD *thd);
   void print(String *str);
   CHARSET_INFO *compare_collation() { return args[0]->collation.collation; }
+  void top_level_item() { abort_on_null=1; }
 };