[HACKERS] about type cast

[zoulx1982]

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.