MDEV-16958 Assertion `field_length < 5' failed in Field_year::val_str or data...

MDEV-16958 Assertion `field_length < 5' failed in Field_year::val_str or data corruption upon SELECT with UNION and aggregate functions

mysql-test/r/type_year.result

@@ -444,5 +444,14 @@ Warnings:
 Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 2010)
 DROP TABLE t1;
 #
+# MDEV-16958 Assertion `field_length < 5' failed in Field_year::val_str or data corruption upon SELECT with UNION and aggregate functions
+#
+CREATE TABLE t1 (f YEAR);
+INSERT IGNORE INTO t1 VALUES (1971),(1972);
+SELECT MAX( NULLIF( f, '1900' ) ) AS f FROM t1 UNION SELECT MAX( NULLIF( f, '1900' ) ) AS f FROM t1;
+f
+1972
+DROP TABLE t1;
+#
 # End of 10.1 tests
 #

mysql-test/t/type_year.test

@@ -214,6 +214,17 @@ SELECT * FROM t1 WHERE a=10 AND a>=10;
 EXPLAIN EXTENDED SELECT * FROM t1 WHERE a=10 AND a>=10;
 DROP TABLE t1;
 
+
+--echo #
+--echo # MDEV-16958 Assertion `field_length < 5' failed in Field_year::val_str or data corruption upon SELECT with UNION and aggregate functions
+--echo #
+
+CREATE TABLE t1 (f YEAR);
+INSERT IGNORE INTO t1 VALUES (1971),(1972);
+SELECT MAX( NULLIF( f, '1900' ) ) AS f FROM t1 UNION SELECT MAX( NULLIF( f, '1900' ) ) AS f FROM t1;
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

sql/item.cc

@@ -9476,7 +9476,10 @@ bool Item_type_holder::join_types(THD *thd, Item *item)
     break;
   }
   default:
-    max_length= MY_MAX(max_length, display_length(item));
+    if (fld_type == MYSQL_TYPE_YEAR)
+      max_length= MY_MAX(max_length, item->max_length);
+    else
+      max_length= MY_MAX(max_length, display_length(item));
   };
   maybe_null|= item->maybe_null;
   get_full_info(item);