Ok, how about
CREATE FUNCTION FOO (one int, two float8 default 3.14, three int[]
default '{6,7,8,90}')....;
and than SELECT FOO( 777, DEFAULT, '{1,2,3,4,5}');
I have no idea what SQL standard says in that case, all I know is that
keyword DEFAULT exists in it, and is used in queries for similar
purpose.