MDEV-26824 Can't add foreign key with empty referenced columns list

create_table_info_t::create_foreign_keys() expects equal number of
iterations through fk->columns and fk->ref_columns. If fk->ref_columns
is empty copy it from fk->columns.

mysql-test/suite/innodb/r/foreign_key.result

@@ -896,4 +896,27 @@ create or replace table t1 (a varchar(4096) unique) engine=innodb;
 create or replace table t2 (pk int primary key, a varchar(4096) unique, foreign key(a) references t1(a) on update cascade) engine=innodb;
 ERROR HY000: Can't create table `test`.`t2` (errno: 150 "Foreign key constraint is incorrectly formed")
 drop table t1;
+#
+# MDEV-26824 Can't add foreign key with empty referenced columns list
+#
+create table t2(a int primary key) engine=innodb;
+create table t1(a int primary key, b int) engine=innodb;
+alter table t2 add foreign key(a) references t1(a, b);
+ERROR 42000: Incorrect foreign key definition for 'foreign key without name': Key reference and table reference don't match
+create or replace table t1(a tinyint primary key) engine innodb;
+alter table t2 add foreign key(a) references t1;
+ERROR HY000: Can't create table `test`.`t2` (errno: 150 "Foreign key constraint is incorrectly formed")
+create or replace table t1(b int primary key) engine innodb;
+alter table t2 add foreign key(a) references t1;
+ERROR HY000: Can't create table `test`.`t2` (errno: 150 "Foreign key constraint is incorrectly formed")
+create or replace table t1(a int primary key, b int) engine innodb;
+alter table t2 add foreign key(a) references t1;
+show create table t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `a` int(11) NOT NULL,
+  PRIMARY KEY (`a`),
+  CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`a`) REFERENCES `t1` (`a`)
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+drop tables t2, t1;
 # End of 10.5 tests

mysql-test/suite/innodb/t/foreign_key.test

@@ -902,6 +902,24 @@ create or replace table t2 (pk int primary key, a varchar(4096) unique, foreign
 
 drop table t1;
 
+--echo #
+--echo # MDEV-26824 Can't add foreign key with empty referenced columns list
+--echo #
+create table t2(a int primary key) engine=innodb;
+create table t1(a int primary key, b int) engine=innodb;
+--error ER_WRONG_FK_DEF
+alter table t2 add foreign key(a) references t1(a, b);
+create or replace table t1(a tinyint primary key) engine innodb;
+--error ER_CANT_CREATE_TABLE
+alter table t2 add foreign key(a) references t1;
+create or replace table t1(b int primary key) engine innodb;
+--error ER_CANT_CREATE_TABLE
+alter table t2 add foreign key(a) references t1;
+create or replace table t1(a int primary key, b int) engine innodb;
+alter table t2 add foreign key(a) references t1;
+show create table t2;
+drop tables t2, t1;
+
 --echo # End of 10.5 tests
 
 --source include/wait_until_count_sessions.inc

sql/sql_table.cc

@@ -3809,15 +3809,19 @@ mysql_prepare_create_table(THD *thd, HA_CREATE_INFO *create_info,
       Foreign_key *fk_key= (Foreign_key*) key;
       if (fk_key->validate(alter_info->create_list))
         DBUG_RETURN(TRUE);
-      if (fk_key->ref_columns.elements &&
-	  fk_key->ref_columns.elements != fk_key->columns.elements)
+      if (fk_key->ref_columns.elements)
       {
-        my_error(ER_WRONG_FK_DEF, MYF(0),
-                 (fk_key->name.str ? fk_key->name.str :
-                                     "foreign key without name"),
-                 ER_THD(thd, ER_KEY_REF_DO_NOT_MATCH_TABLE_REF));
-	DBUG_RETURN(TRUE);
+        if (fk_key->ref_columns.elements != fk_key->columns.elements)
+        {
+          my_error(ER_WRONG_FK_DEF, MYF(0),
+                  (fk_key->name.str ? fk_key->name.str :
+                                      "foreign key without name"),
+                  ER_THD(thd, ER_KEY_REF_DO_NOT_MATCH_TABLE_REF));
+          DBUG_RETURN(TRUE);
+        }
       }
+      else
+        fk_key->ref_columns.append(&fk_key->columns);
       continue;
     }
     (*key_count)++;