FOREIGN KEY can reference any KEY, not just PRIMARY KEY and UNIQUE

A foreign key should only be able to reference a UNIQUE or PRIMARY KEY constraint, but MariaDB allows any key/index to be referenced. To reproduce:

CREATE TABLE t1(c1 INT NOT NULL PRIMARY KEY,

  c2 INT NOT NULL,

  KEY(c2));

CREATE TABLE t2(c1 INT NOT NULL PRIMARY KEY,

  c2 INT NOT NULL,

  FOREIGN KEY (c2) REFERENCES t1(c2));

This is invalid in the SQL Standard as well as according to MariaDB Knowledge base: https://mariadb.com/kb/en/library/foreign-keys/ which states "The referenced columns must be a PRIMARY KEY or a UNIQUE index".