BUG #4739: strange behavior of EXTRACT(quarter from INTERVAL)

The following bug has been logged online:

Bug reference:      4739
Logged by:          Roman Kononov
Email address:      kononov@ftml.net
PostgreSQL version: 8.3.7
Operating system:   GNU/Linux x86_64
Description:        strange behavior of EXTRACT(quarter from INTERVAL)
Details:

test=# select month, extract(quarter from (month::varchar || '
month')::interval) from tbl;
 month | date_part
-------+-----------
    -6 |        -1
    -5 |         0
    -4 |         0
    -3 |         0
    -2 |         1
    -1 |         1
     0 |         1
     1 |         1
     2 |         1
     3 |         2
     4 |         2
     5 |         2
     6 |         3
(13 rows)