Fixed bug mdev-4942.

Made sure that degenerate conjunctions/disjunctions are obtained from
AND/OR conditions.

mysql-test/r/join_outer.result

@@ -1836,4 +1836,16 @@ SELECT a.* FROM t1 a LEFT JOIN t1 b ON a.id = b.id
 WHERE a.modified > b.modified or b.modified IS NULL;
 id	modified
 DROP TABLE t1;
+#
+# Bug mdev-4942: LEFT JOIN with conjunctive 
+#                <non-nullable datetime field> IS NULL in WHERE 
+#                causes an assert failure  
+#
+CREATE TABLE t1 ( i1 int, d1 date );
+INSERT INTO t1 VALUES (1,'2001-06-26'), (2,'2000-11-16');
+CREATE TABLE t2 ( i2 int, d2 date NOT NULL );
+INSERT INTO t2 VALUES (3,'2000-03-06'), (4,'2007-09-25');
+SELECT * FROM t1 LEFT JOIN t2 ON i1 = i2 WHERE d1 IS NULL AND d2 IS NULL;
+i1	d1	i2	d2
+DROP TABLE t1,t2;
 SET optimizer_switch=@save_optimizer_switch;

mysql-test/r/join_outer_jcl6.result

@@ -1847,6 +1847,18 @@ SELECT a.* FROM t1 a LEFT JOIN t1 b ON a.id = b.id
 WHERE a.modified > b.modified or b.modified IS NULL;
 id	modified
 DROP TABLE t1;
+#
+# Bug mdev-4942: LEFT JOIN with conjunctive 
+#                <non-nullable datetime field> IS NULL in WHERE 
+#                causes an assert failure  
+#
+CREATE TABLE t1 ( i1 int, d1 date );
+INSERT INTO t1 VALUES (1,'2001-06-26'), (2,'2000-11-16');
+CREATE TABLE t2 ( i2 int, d2 date NOT NULL );
+INSERT INTO t2 VALUES (3,'2000-03-06'), (4,'2007-09-25');
+SELECT * FROM t1 LEFT JOIN t2 ON i1 = i2 WHERE d1 IS NULL AND d2 IS NULL;
+i1	d1	i2	d2
+DROP TABLE t1,t2;
 SET optimizer_switch=@save_optimizer_switch;
 set join_cache_level=default;
 show variables like 'join_cache_level';

mysql-test/t/join_outer.test

@@ -1376,4 +1376,20 @@ SELECT a.* FROM t1 a LEFT JOIN t1 b ON a.id = b.id
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug mdev-4942: LEFT JOIN with conjunctive 
+--echo #                <non-nullable datetime field> IS NULL in WHERE 
+--echo #                causes an assert failure  
+--echo #
+
+CREATE TABLE t1 ( i1 int, d1 date );
+INSERT INTO t1 VALUES (1,'2001-06-26'), (2,'2000-11-16');
+
+CREATE TABLE t2 ( i2 int, d2 date NOT NULL );
+INSERT INTO t2 VALUES (3,'2000-03-06'), (4,'2007-09-25');
+
+SELECT * FROM t1 LEFT JOIN t2 ON i1 = i2 WHERE d1 IS NULL AND d2 IS NULL;
+
+DROP TABLE t1,t2;
+
 SET optimizer_switch=@save_optimizer_switch;

sql/sql_select.cc

@@ -13350,7 +13350,8 @@ remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value)
           In these cases the disjunct/conjunct must be merged into the
           argument list of cond.
 	*/
-        if (new_item->type() == Item::COND_ITEM)
+        if (new_item->type() == Item::COND_ITEM &&
+            item->type() == Item::COND_ITEM)
         {
           DBUG_ASSERT(((Item_cond *) cond)->functype() == 
                       ((Item_cond *) new_item)->functype());