BUG #5237: strange int->bit and bit->int conversions

The following bug has been logged online:

Bug reference:      5237
Logged by:          Roman Kononov
Email address:      kononov@ftml.net
PostgreSQL version: 8.4.1
Operating system:   GNU/Linux x86_64
Description:        strange int->bit and bit->int conversions
Details:

test=# select (11::int4<<23 | 11::int4)::bit(32);
 00000101100000000000000000001011

test=# select (11::int4<<23 | 11::int4)::bit(33);
 000001011100000000000000000001011

test=# select (11::int4<<23 | 11::int4)::bit(39);
 000001010000101100000000000000000001011

test=# select (11::int4<<23 | 11::int4)::bit(40);
 0000000000000101100000000000000000001011

The ::bit(33) and ::bit(39) conversions seem wrong.

test-std=# select 1::int4::bit(32)::int4;
    1

test-std=# select 1::int4::bit(33)::int4;
ERROR:  integer out of range

Why is it out of range?