Patch for https://bugzilla.redhat.com/show_bug.cgi?id=1686818
Picked from https://github.com/MariaDB/server/commit/802ce9672f

Fixed by rebase to 10.3.12

--- mariadb-10.3.11/storage/innobase/handler/handler0alter.cc	2018-11-16 19:04:53.000000000 +0100
+++ mariadb-10.3.11/storage/innobase/handler/handler0alter.cc_patched	2019-03-13 11:52:15.752268825 +0100
@@ -7712,36 +7712,40 @@ err_exit:
 	     index != NULL;
 	     index = dict_table_get_next_index(index)) {
 
+		bool has_prefixes = false;
+		for (size_t i = 0; i < dict_index_get_n_fields(index); i++) {
+			if (dict_index_get_nth_field(index, i)->prefix_len) {
+				has_prefixes = true;
+				break;
+			}
+		}
+
 		for (ulint i = 0; i < dict_index_get_n_fields(index); i++) {
-			if (my_strcasecmp(
-				system_charset_info,
-				dict_index_get_nth_field(index, i)->name,
-				from)) {
+			const dict_field_t* field
+			    = dict_index_get_nth_field(index, i);
+			if (my_strcasecmp(system_charset_info, field->name,
+					  from)) {
 				continue;
 			}
 
 			info = pars_info_create();
 
+			int pos = i;
+			if (has_prefixes) {
+				pos = (pos << 16) + field->prefix_len;
+			}
+
 			pars_info_add_ull_literal(info, "indexid", index->id);
-			pars_info_add_int4_literal(info, "nth", i);
+			pars_info_add_int4_literal(info, "nth", pos);
 			pars_info_add_str_literal(info, "new", to);
 
 			error = que_eval_sql(
 				info,
 				"PROCEDURE RENAME_SYS_FIELDS_PROC () IS\n"
 				"BEGIN\n"
-
 				"UPDATE SYS_FIELDS SET COL_NAME=:new\n"
 				"WHERE INDEX_ID=:indexid\n"
 				"AND POS=:nth;\n"
-
-				/* Try again, in case there is a prefix_len
-				encoded in SYS_FIELDS.POS */
-
-				"UPDATE SYS_FIELDS SET COL_NAME=:new\n"
-				"WHERE INDEX_ID=:indexid\n"
-				"AND POS>=65536*:nth AND POS<65536*(:nth+1);\n"
-
 				"END;\n",
 				FALSE, trx);
 
--- mariadb-10.3.11/mysql-test/suite/innodb/r/innodb-alter.result	2018-11-16 19:04:50.000000000 +0100
+++ mariadb-10.3.11/mysql-test/suite/innodb/r/innodb-alter.result_patched	2019-03-13 12:06:48.788107502 +0100
@@ -879,6 +879,27 @@ NAME
 a
 b
 DROP TABLE t1;
+# and an MDEV-18041 regression related to indexes prefixes
+create table `test` (
+`test_old` varchar(255) NOT NULL,
+`other` varchar(255) NOT NULL,
+PRIMARY KEY (`test_old`,`other`),
+UNIQUE KEY uk (`test_old`(100), `other`)
+) ENGINE=InnoDB;
+select name, pos from information_schema.innodb_SYS_FIELDS where name in ('test_old', 'other', 'test_new');
+name	pos
+test_old	0
+other	1
+test_old	0
+other	1
+alter table `test` CHANGE COLUMN `test_old` `test_new` varchar(255) NOT NULL;
+select name, pos from information_schema.innodb_SYS_FIELDS where name in ('test_old', 'other', 'test_new');
+name	pos
+test_new	0
+other	1
+test_new	0
+other	1
+drop table `test`;
 #
 # BUG 20029625 - HANDLE_FATAL_SIGNAL (SIG=11) IN
 #                DICT_MEM_TABLE_COL_RENAME_LOW
--- mariadb-10.3.11/mysql-test/suite/innodb/t/innodb-alter.test	2018-11-16 19:04:50.000000000 +0100
+++ mariadb-10.3.11/mysql-test/suite/innodb/t/innodb-alter.test_patched	2019-03-13 12:08:36.398847184 +0100
@@ -545,6 +545,19 @@ SELECT C.NAME FROM INFORMATION_SCHEMA.IN
   WHERE T.NAME='test/t1';
 DROP TABLE t1;
 
+--echo # and an MDEV-18041 regression related to indexes prefixes
+create table `test` (
+  `test_old` varchar(255) NOT NULL,
+  `other` varchar(255) NOT NULL,
+  PRIMARY KEY (`test_old`,`other`),
+  UNIQUE KEY uk (`test_old`(100), `other`)
+) ENGINE=InnoDB;
+
+select name, pos from information_schema.innodb_SYS_FIELDS where name in ('test_old', 'other', 'test_new');
+alter table `test` CHANGE COLUMN `test_old` `test_new` varchar(255) NOT NULL;
+select name, pos from information_schema.innodb_SYS_FIELDS where name in ('test_old', 'other', 'test_new');
+drop table `test`;
+
 
 --echo #
 --echo # BUG 20029625 - HANDLE_FATAL_SIGNAL (SIG=11) IN