fixed subselect null test

fixed method definition (overloading instead of redefinition)
fixed time expression type definition

mysql-test/r/subselect.result

@@ -47,6 +47,11 @@ select (SELECT 1 FROM (SELECT 1) a PROCEDURE ANALYSE(1));
 Wrong usage of PROCEDURE and subquery
 SELECT 1 FROM (SELECT 1) a PROCEDURE ANALYSE((SELECT 1));
 Incorrect parameters to procedure 'ANALYSE'
+SELECT (SELECT 1) as a FROM (SELECT 1) b WHERE (SELECT a) IS NULL;
+a
+SELECT (SELECT 1) as a FROM (SELECT 1) b WHERE (SELECT a) IS NOT NULL;
+a
+1
 drop table if exists t1,t2,t3,t4,t5,t6,t7,t8;
 create table t1 (a int);
 create table t2 (a int, b int);

mysql-test/t/subselect.test

@@ -24,6 +24,8 @@ SELECT 1 FROM (SELECT 1 as a) b WHERE 1 IN (SELECT (SELECT a));
 select (SELECT 1 FROM (SELECT 1) a PROCEDURE ANALYSE(1));
 -- error 1108
 SELECT 1 FROM (SELECT 1) a PROCEDURE ANALYSE((SELECT 1));
+SELECT (SELECT 1) as a FROM (SELECT 1) b WHERE (SELECT a) IS NULL;
+SELECT (SELECT 1) as a FROM (SELECT 1) b WHERE (SELECT a) IS NOT NULL;
 
 drop table if exists t1,t2,t3,t4,t5,t6,t7,t8;
 create table t1 (a int);

sql/item.h

@@ -208,7 +208,7 @@ public:
   {
     return field->result_type();
   }
-  enum_field_types field_type()
+  enum_field_types field_type() const
   {
     return field->type();
   }

sql/item_subselect.h

@@ -75,7 +75,11 @@ public:
   bool assigned() { return value_assigned; }
   void assigned(bool a) { value_assigned= a; }
   enum Type type() const;
-  bool is_null() { return null_value; }
+  bool is_null()
+  {
+    val_int();
+    return null_value;
+  }
   bool fix_fields(THD *thd, TABLE_LIST *tables, Item **ref);
   virtual void fix_length_and_dec();
   table_map used_tables() const;

sql/item_timefunc.cc

@@ -1025,7 +1025,7 @@ void Item_date_add_interval::fix_length_and_dec()
     cached_field_type= MYSQL_TYPE_DATETIME;
   else if (arg0_field_type == MYSQL_TYPE_DATE)
   {
-    if (int_type <= INTERVAL_MONTH || int_type == INTERVAL_YEAR_MONTH)
+    if (int_type <= INTERVAL_DAY || int_type == INTERVAL_YEAR_MONTH)
       cached_field_type= arg0_field_type;
     else
       cached_field_type= MYSQL_TYPE_DATETIME;