MDEV-7816 ALTER with DROP INDEX and ADD INDEX .. COMMENT='comment2' ignores the new comment

Consider two indexes different if their comments differ

mysql-test/r/alter_table.result

@@ -2007,3 +2007,13 @@ INSERT INTO t1 VALUES (1),(2),(3),(4),(5),(6),(7),(8);
 INSERT INTO t1 SELECT a.* FROM t1 a, t1 b, t1 c, t1 d, t1 e;
 ALTER TABLE t1 MODIFY i FLOAT;
 DROP TABLE t1;
+CREATE TABLE t1(a INT);
+CREATE INDEX i1 ON t1(a) COMMENT 'comment1';
+ALTER TABLE t1 DROP INDEX i1, ADD INDEX i1(a) COMMENT 'comment2';
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  KEY `i1` (`a`) COMMENT 'comment2'
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;

mysql-test/t/alter_table.test

@@ -1697,3 +1697,11 @@ INSERT INTO t1 SELECT a.* FROM t1 a, t1 b, t1 c, t1 d, t1 e;
 ALTER TABLE t1 MODIFY i FLOAT;
 DROP TABLE t1;
 
+#
+# MDEV-7816 ALTER with DROP INDEX and ADD INDEX .. COMMENT='comment2' ignores the new comment
+#
+CREATE TABLE t1(a INT);
+CREATE INDEX i1 ON t1(a) COMMENT 'comment1';
+ALTER TABLE t1 DROP INDEX i1, ADD INDEX i1(a) COMMENT 'comment2';
+SHOW CREATE TABLE t1;
+DROP TABLE t1;

sql/sql_table.cc

@@ -6368,6 +6368,13 @@ static bool fill_alter_inplace_info(THD *thd,
           new_field->field->field_index != key_part->fieldnr - 1)
         goto index_changed;
     }
+
+    /* Check that key comment is not changed. */
+    if (table_key->comment.length != new_key->comment.length ||
+        (table_key->comment.length &&
+         strcmp(table_key->comment.str, new_key->comment.str) != 0))
+        goto index_changed;
+
     continue;
 
   index_changed: