MDEV-21841 CONV() function doesn't truncate its output to 21 when uses default charset.

mysql-test/main/func_str.result

@@ -5159,6 +5159,21 @@ DROP TABLE t1, t2;
 # Start of 10.4 tests
 #
 #
+# MDEV-21841 CONV() function truncates the result type to 21 symbol.
+#
+CREATE TABLE t1(i BIGINT);
+INSERT INTO t1 VALUES (-1);
+CREATE TABLE t2 AS SELECT conv(i,16,2) from t1;
+SELECT * FROM t2;
+conv(i,16,2)
+1111111111111111111111111111111111111111111111111111111111111111
+SHOW CREATE TABLE t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `conv(i,16,2)` varchar(64) CHARACTER SET utf8 DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1,t2;
+#
 # MDEV-18205 Assertion `str_length < len' failed in Binary_string::realloc_raw
 #
 SELECT GROUP_CONCAT( UpdateXML( '<a>new year</a>', '/a', '2019-01-01 00:00:00' ), ENCODE('text','pass') ) AS f;

mysql-test/main/func_str.test

@@ -2099,6 +2099,17 @@ DROP TABLE t1, t2;
 --echo # Start of 10.4 tests
 --echo #
 
+--echo #
+--echo # MDEV-21841 CONV() function truncates the result type to 21 symbol.
+--echo #
+
+CREATE TABLE t1(i BIGINT);
+INSERT INTO t1 VALUES (-1);
+CREATE TABLE t2 AS SELECT conv(i,16,2) from t1;
+SELECT * FROM t2;
+SHOW CREATE TABLE t2;
+DROP TABLE t1,t2;
+
 --echo #
 --echo # MDEV-18205 Assertion `str_length < len' failed in Binary_string::realloc_raw
 --echo #

sql/item_strfunc.h

@@ -1205,7 +1205,7 @@ class Item_func_conv :public Item_str_func
   bool fix_length_and_dec()
   {
     collation.set(default_charset());
-    max_length=64;
+    fix_char_length(64);
     maybe_null= 1;
     return FALSE;
   }