I found following problem:
postgres=# create or replace function sum_items(anyarray) returns
anyelement as $$ select sum($1[i]) from
generate_series(array_lower($1,1), array_upper($1,1)) g(i)$$ language
sql immutable;
CREATE FUNCTION
Time: 1,983 ms
postgres=# select sum_items(array[1,2,3]);
ERROR: return type mismatch in function declared to return integer
DETAIL: Actual return type is bigint.
CONTEXT: SQL function "sum_items" during startup
postgres=#
so in this case we need explicit casting to result type - like
create or replace function sum_items(anyarray) returns anyelement as $$ select sum($1[i])::anyelement -- <<<<<< from
generate_series(array_lower($1,1),array_upper($1,1)) g(i)
$$ language sql immutable;
or some flag that ensure explicit casting.
Regards
Pavel Stehule