Bug#31291 ALTER TABLE CONVERT TO CHARACTER SET does not change some data types

added ability for TINY[MEDIUM] text fields 
to be converted to greater subtype during
alter if necessary(altered charset)

mysql-test/r/alter_table.result

@@ -915,3 +915,19 @@ check table t1;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
 drop table t1;
+create table t1 (a tinytext character set latin1);
+alter table t1 convert to character set utf8;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` text
+) ENGINE=MyISAM DEFAULT CHARSET=utf8
+drop table t1;
+create table t1 (a mediumtext character set latin1);
+alter table t1 convert to character set utf8;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` longtext
+) ENGINE=MyISAM DEFAULT CHARSET=utf8
+drop table t1;

mysql-test/t/alter_table.test

@@ -696,3 +696,16 @@ unlock tables;
 select * from t1;
 check table t1;
 drop table t1;
+
+
+#
+# Bug#31291 ALTER TABLE CONVERT TO CHARACTER SET does not change some data types
+#
+create table t1 (a tinytext character set latin1);
+alter table t1 convert to character set utf8;
+show create table t1;
+drop table t1;
+create table t1 (a mediumtext character set latin1);
+alter table t1 convert to character set utf8;
+show create table t1;
+drop table t1;

sql/sql_table.cc

@@ -1535,7 +1535,9 @@ static bool prepare_blob_field(THD *thd, create_field *sql_field)
     
   if ((sql_field->flags & BLOB_FLAG) && sql_field->length)
   {
-    if (sql_field->sql_type == FIELD_TYPE_BLOB)
+    if (sql_field->sql_type == FIELD_TYPE_BLOB ||
+        sql_field->sql_type == FIELD_TYPE_TINY_BLOB ||
+        sql_field->sql_type == FIELD_TYPE_MEDIUM_BLOB)
     {
       /* The user has given a length to the blob column */
       sql_field->sql_type= get_blob_type_from_length(sql_field->length);