cannot prefix a SQL UDF to make it an aggregate function

[stdrc]

Describe the bug

create function as2 ( int[] ) returns bigint language sql as 'select array_sum($1)';
select aggregate:as2(a) from (values (1), (2)) t(a);

output:

ERROR:  Failed to run the query

Caused by these errors (recent errors listed first):
  1: failed to build executor
  2: Expr error
  3: Failed to decode prost: field not found `identifier`

expected:

3