Test for bug #53820 "ALTER a MEDIUMINT column table causes full

table copy".

This patch only adds test case as the bug itself was addressed 
by Ramil's fix for bug 50946 "fast index creation still seems
to copy the table".

mysql-test/r/alter_table.result

@@ -1370,3 +1370,16 @@ CREATE TABLE t1 (id int);
 INSERT INTO t1 VALUES (1), (2);
 ALTER TABLE t1 ADD COLUMN (f1 INT), ADD COLUMN (f2 INT), ADD KEY f2k(f2);
 DROP TABLE t1;
+#
+# Test for bug #53820 "ALTER a MEDIUMINT column table causes full
+#                      table copy".
+#
+DROP TABLE IF EXISTS t1;
+CREATE TABLE t1 (a INT, b MEDIUMINT);
+INSERT INTO t1 VALUES (1, 1), (2, 2);
+# The below ALTER should not copy table and so no rows should
+# be shown as affected.
+ALTER TABLE t1 CHANGE a id INT;
+affected rows: 0
+info: Records: 0  Duplicates: 0  Warnings: 0
+DROP TABLE t1;

mysql-test/t/alter_table.test

@@ -1128,3 +1128,19 @@ INSERT INTO t1 VALUES (1), (2);
 ALTER TABLE t1 ADD COLUMN (f1 INT), ADD COLUMN (f2 INT), ADD KEY f2k(f2);
 DROP TABLE t1;
 
+
+--echo #
+--echo # Test for bug #53820 "ALTER a MEDIUMINT column table causes full
+--echo #                      table copy".
+--echo #
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+--enable_warnings
+CREATE TABLE t1 (a INT, b MEDIUMINT);
+INSERT INTO t1 VALUES (1, 1), (2, 2);
+--echo # The below ALTER should not copy table and so no rows should
+--echo # be shown as affected.
+--enable_info
+ALTER TABLE t1 CHANGE a id INT;
+--disable_info
+DROP TABLE t1;