Expression transform error

spark.sql(
  """
    |select to_date(a) a, to_date(b) b from
    |(select  a, a as b from
    |(select to_date(a) a from
    | values ('2020-02-01') as t1(a)
    | group by to_date(a)) t3
    |union all
    |select a, b from
    |(select to_date(a) a, to_date(b) b from
    |values ('2020-01-01','2020-01-02') as t1(a, b)
    | group by to_date(a), to_date(b)) t4) t5
    |group by to_date(a), to_date(b)
    |""".stripMargin).show()