Index usage with differing string types

I stumbled upon this behaviour when digging into the performance of some merge statements generated by hibernate.

Looking at different String types (varchar, text and bpchar) in some cases an index is used when the index type differs from the type in the query, in some cases it isn't used.

Given a table with an index on a bpchar column:

create table test (id bpchar(8) primary key);

Both of the following queries use the index:

explain select * from test where id = 'foo'::bpachar(3);