Postgres unique constraints are checked immediately not at the end of the statement

postgres-immediate-unique-constraint.sql

CREATE TABLE test (
  val        TEXT,
  sort_order INTEGER NOT NULL UNIQUE
);

-- works fine:
TRUNCATE test;
INSERT INTO test
VALUES ('B', 2), 
       ('A', 1);

UPDATE test
SET sort_order = sort_order + 1;

-- UPDATE fails:
TRUNCATE test;
INSERT INTO test
VALUES ('A', 1), 
       ('B', 2);

UPDATE test
SET sort_order = sort_order + 1;

-- result:
-- ERROR: duplicate key value violates unique constraint "test_sort_order_key" 
--        Detail: Key (sort_order)=(2) already exists.