Dimension_group w/ type "time" doesn't work with datatype: datetime for BigQuery

[vfx1]

Consider the following code:

view: datetime {
  derived_table: {
    sql: SELECT CURRENT_DATETIME as dt;; # Bigquery dialect
  }

  dimension_group: dt {
    type: time
    timeframes: [
      date,
      month
    ]
    sql: ${TABLE}.dt ;;
    datatype: datetime
  }
}

explore: datetime {}

This is supposed to be pretty straightforward - a dimension group with datetime as underlying data type.

Looker generates the following SQL:

WITH datetime AS (SELECT CURRENT_DATETIME as dt)
SELECT 
	CAST(TIMESTAMP(FORMAT_TIMESTAMP('%F %T', datetime.dt , 'Asia/Kolkata')) AS DATE) AS datetime_dt_date
FROM datetime

GROUP BY 1

Which produces an error as the underlying type is not timestamp.

Failed to retrieve data - No matching signature for function FORMAT_TIMESTAMP for argument types: STRING, DATETIME, STRING. Supported signature: FORMAT_TIMESTAMP(STRING, TIMESTAMP, [STRING]) at [4:24]

This happens with other datatypes too. There is a way to overcome it by casting the datetime to timestamp in the ‘sql:’ element but this is a hack and produces redundant transformation.

Is it a bug?