about type cast

hi，

there is a problem about type cast that i don't understand, follow is my test.

 

postgres=# select 10::bit(3);
 bit
-----
 010
(1 row)

postgres=# select 10::bit varying(3);
ERROR:  cannot cast type integer to bit varying
LINE 1: select 10::bit varying(3);
                 ^
postgres=#

 

my question is why int can cast to bit , but bot for bit varying?

i want to know the reason.

thank you for your timing.