Cast possible only throught a function

Hi all,
running on Postgres 7.3.3
I seen that cast a integer to an intervall is not permitted

select 0::interval;    <- give error

but

select 0::text::interval;   <- works fine

but I notice that the following function return correctly
doing the integer -> interval cast in one "shot":

create or replace function foo()
returns interval as '
declare
begin
 return 0;
end;
' language 'plpgsql';



this implicit cast could not be source of issue ?


Gaetano