MDEV-14221 Assertion `0' failed in Item::field_type_for_temporal_comparison

Item_null_result did not override type_handler() because of a wrong merge
of d8a9b524 (MDEV-14221) from 10.1.

Overriding type_handler().

Removing the old style field_type() method. It's not relevant any more.

mysql-test/main/type_datetime_hires.result

@@ -889,5 +889,16 @@ Warning	1411	Incorrect date value: '0000-02-28' for function round(datetime)
 Warning	1411	Incorrect date value: '0000-12-31' for function round(datetime)
 DROP TABLE t1;
 #
+# MDEV-20984 Possibly wrong result or Assertion `args[0]->type_handler()->mysql_timestamp_type() == MYSQL_TIMESTAMP_DATETIME' failed in Item_func_round::date_op
+#
+CREATE TABLE t1 (a DATETIME);
+INSERT INTO t1 VALUES ('1979-01-03 10:33:32'),('2012-12-12 12:12:12');
+SELECT ROUND(a) AS f FROM t1 GROUP BY a WITH ROLLUP;
+f
+1979-01-03 10:33:32
+2012-12-12 12:12:12
+NULL
+DROP TABLE t1;
+#
 # End of 10.4 tests
 #

mysql-test/main/type_datetime_hires.test

@@ -146,6 +146,16 @@ SELECT a, ROUND(a,-6) FROM t1;
 
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-20984 Possibly wrong result or Assertion `args[0]->type_handler()->mysql_timestamp_type() == MYSQL_TIMESTAMP_DATETIME' failed in Item_func_round::date_op
+--echo #
+
+CREATE TABLE t1 (a DATETIME);
+INSERT INTO t1 VALUES ('1979-01-03 10:33:32'),('2012-12-12 12:12:12');
+SELECT ROUND(a) AS f FROM t1 GROUP BY a WITH ROLLUP;
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

sql/item.h

@@ -3654,9 +3654,11 @@ class Item_null_result :public Item_null
   Field *result_field;
   Item_null_result(THD *thd): Item_null(thd), result_field(0) {}
   bool is_result_field() { return result_field != 0; }
-  enum_field_types field_type() const
+  const Type_handler *type_handler() const
   {
-    return result_field->type();
+    if (result_field)
+      return result_field->type_handler();
+    return &type_handler_null;
   }
   Field *create_tmp_field_ex(TABLE *table, Tmp_field_src *src,
                              const Tmp_field_param *param)