Bug #55565: debug assertion when ordering by expressions with user

variable assignments

The assert() that is firing is checking if expressions that can't be
null return a NULL when evaluated.
MAKEDATE() function can return NULL if the second argument is 
less then or equal to 0. Thus its nullability depends not only on 
the nullability of its arguments but also on their values.
Fixed by (overoptimistically) setting MAKEDATE() to be nullable 
despite the nullability of its arguments.
Test added.
Had to update one test result to reflect the metadata change.

mysql-test/r/func_sapdb.result

@@ -194,7 +194,7 @@ date("1997-12-31 23:59:59.000001") as f8,
 time("1997-12-31 23:59:59.000001") as f9;
 describe t1;
 Field	Type	Null	Key	Default	Extra
-f1	date	NO		0000-00-00	
+f1	date	YES		NULL	
 f2	datetime	YES		NULL	
 f3	time	YES		NULL	
 f4	time	YES		NULL	

mysql-test/r/func_time.result

@@ -1335,4 +1335,12 @@ date_sub("0069-01-01 00:00:01",INTERVAL 2 SECOND)
 select date_sub("0169-01-01 00:00:01",INTERVAL 2 SECOND);
 date_sub("0169-01-01 00:00:01",INTERVAL 2 SECOND)
 0168-12-31 23:59:59
+CREATE TABLE t1(a DOUBLE NOT NULL);
+INSERT INTO t1 VALUES (0),(9.216e-096);
+# should not crash
+SELECT 1 FROM t1 ORDER BY @x:=makedate(a,a);
+1
+1
+1
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/func_time.test

@@ -838,4 +838,15 @@ select date_sub("0069-01-01 00:00:01",INTERVAL 2 SECOND);
 select date_sub("0169-01-01 00:00:01",INTERVAL 2 SECOND);
 
 
+#
+# Bug #55565: debug assertion when ordering by expressions with user
+# variable assignments
+#
+
+CREATE TABLE t1(a DOUBLE NOT NULL);
+INSERT INTO t1 VALUES (0),(9.216e-096);
+--echo # should not crash
+SELECT 1 FROM t1 ORDER BY @x:=makedate(a,a);
+DROP TABLE t1;
+
 --echo End of 5.1 tests

sql/item_timefunc.h

@@ -881,6 +881,8 @@ class Item_func_makedate :public Item_date_func
   { 
     decimals=0;
     max_length=MAX_DATE_WIDTH*MY_CHARSET_BIN_MB_MAXLEN;
+    /* It returns NULL when the second argument is less or equal to 0 */
+    maybe_null= 1;
   }
   longlong val_int();
 };