I am getting cache lookup failed for constraint error on master and 11beta3 with below test case.
[edb@localhost bin]$ ./psql postgres psql (11beta3) Type "help" for help.
postgres=# CREATE TABLE non_part (a INT,PRIMARY KEY(a)); CREATE TABLE postgres=# CREATE TABLE part (a INT REFERENCES non_part(a)) PARTITION BY RANGE(a); CREATE TABLE postgres=# CREATE TABLE part_p1 PARTITION OF part FOR VALUES FROM (MINVALUE) TO (MAXVALUE); CREATE TABLE postgres=# ALTER TABLE non_part ALTER COLUMN a TYPE bigint; ERROR: cache lookup failed for constraint 16398