PostgreSQL - CAST vs :: operator on LATERAL table function

While I can

SELECT
  elem, elem
FROM
  (
    VALUES ('1,2'::TEXT)
  ) AS q(arr),
  LATERAL CAST(String_To_Array(q.arr, ',') AS INT[]) AS elem
;

using an explicit call to CAST, I *can't*

SELECT
  elem, elem
FROM
  (
    VALUES ('1,2'::TEXT)
  ) AS q(arr),
  LATERAL String_To_Array(q.arr, ',')::INT[] AS elem
;

using the implicitly calling :: operator: > ERROR: syntax error at or near "::" One other location at which an explicit CAST is required:

CREATE INDEX ON ... ( CAST( AS ) );

` I doubt there is a syntactical reason, e.g. using extra enclosing parenthesis - which is incorrect here. Is the explicit function call simply needed at this point as part of the low level implementation? Or does it follow any language rules?