LP993103: Wrong result with LAST_DAY('0000-00-00 00:00:00')IS NULL in WHERE condition

Fix is to set  maybe_null  flag for Item_func_last_day.

mysql-test/r/type_date.result

@@ -21,6 +21,12 @@ name	cdate	note
 name1	1998-01-01	note01
 name2	1998-01-01	note01
 drop table t1,t2;
+CREATE TABLE t1(a INT);
+INSERT INTO t1 VALUES(1);
+SELECT * FROM t1 WHERE LAST_DAY('0000-00-00 00:00:00') IS NULL;
+a
+1
+DROP TABLE t1;
 CREATE TABLE t1 ( datum DATE );
 INSERT INTO t1 VALUES ( "2000-1-1" );
 INSERT INTO t1 VALUES ( "2000-1-2" );

mysql-test/t/type_date.test

@@ -26,6 +26,13 @@ INSERT INTO t2 VALUES ('1998-01-02','note02');
 select name,t1.cdate,note from t1,t2 where t1.cdate=t2.cdate and t1.cdate='1998-01-01';
 drop table t1,t2;
 
+# MariaDB lp:993103. WHERE LAST_DAY(zero_date) IS NULL does not evaluate to TRUE.
+
+CREATE TABLE t1(a INT);
+INSERT INTO t1 VALUES(1);
+SELECT * FROM t1 WHERE LAST_DAY('0000-00-00 00:00:00') IS NULL;
+DROP TABLE t1;
+
 #
 # Date and BETWEEN
 #

sql/item_timefunc.h

@@ -1169,6 +1169,11 @@ class Item_func_last_day :public Item_date
 {
 public:
   Item_func_last_day(Item *a) :Item_date(a) {}
+  void fix_length_and_dec()
+  {
+    Item_date::fix_length_and_dec();
+    maybe_null= 1;
+  }
   const char *func_name() const { return "last_day"; }
   bool get_date(MYSQL_TIME *res, uint fuzzy_date);
 };