BUG #8393: "ERROR: failed to locate grouping columns" on grouping by varchar returned from function

The following bug has been logged on the website:

Bug reference: 8393
Logged by: Evan Martin
Email address: postgresql(at)realityexists(dot)net
PostgreSQL version: 9.2.4
Operating system: Windows 7 x64 SP1
Description:

version(): PostgreSQL 9.2.4, compiled by Visual C++ build 1600, 64-bit

Run the following:

CREATE OR REPLACE FUNCTION test_group_by()
RETURNS TABLE (my_col varchar(5))
AS $BODY$
SELECT 'hello'::varchar(5);
$BODY$ LANGUAGE sql STABLE;

SELECT my_col
FROM test_group_by()
GROUP BY 1;

Expected result:

'hello'

Actual result:

ERROR: failed to locate grouping columns

Interestingly, if the function is marked "VOLATILE" it works. Casting the
result to "text" also makes it work.