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