BUG #5563: Odd behavior with aggregate_func(DISTINCT foo ORDER BY foo)

The following bug has been logged online:

Bug reference:      5563
Logged by:          Daniel Grace
Email address:      dgrace@wingsnw.com
PostgreSQL version: 9.0beta3
Operating system:   Windows XP 32-bit
Description:        Odd behavior with aggregate_func(DISTINCT foo ORDER BY
foo)
Details:

The manual states:
"If DISTINCT is specified in addition to an order_by_clause, then all the
ORDER BY expressions must match regular arguments of the aggregate; that is,
you cannot sort on an expression that is not included in the DISTINCT  list.
"

However, in some circumstances Postgres will fail

DROP TABLE IF EXISTS foo;
CREATE TABLE foo (
    t VARCHAR
);

INSERT INTO foo (t) VALUES ('a'), ('a'), ('b'), ('b'), ('c');

SELECT STRING_AGG(DISTINCT t::TEXT ORDER BY t::TEXT) FROM foo;