build_table_filename() doesn't support temporary tables. Indeed, it always builds the path as datadir/db_name/table_name, while temporary tables are in the tmpdir. This shows up in various DDL statements that use temporary tables:

create temporary table t1 (a int);

rename table t1 to t2;

This fails. Or

create temporary table t1(a int not null primary key, b int, key(b)) engine=innodb;

--error 1005

alter table t1 add foreign key(b) references t1(a);

drop table t1;

--source include/restart_mysqld.inc

This test case succeeds but leaves an orphant temporary frm in the tmpdir (and it causes a warning on restart).

Note: when this is fixed, please uncomment the corresponding test in innodb-fk-warnings.test