BUG #4718: "select +'8:00'::interval;" gives an error

The following bug has been logged online:

Bug reference:      4718
Logged by:          Roman Kononov
Email address:      kononov@ftml.net
PostgreSQL version: 8.3.7
Operating system:   GNU/Linux x86_64
Description:        "select +'8:00'::interval;" gives an error
Details:

test=# select +'8:00'::interval;
ERROR:  operator does not exist: + interval
LINE 1: select +'8:00'::interval;
               ^
HINT:  No operator matches the given name and argument type(s). You might
need to add explicit type casts.

test=# select -'8:00'::interval;
 -08:00:00