Computed columns and CTEs sometimes lead to missing column

[Fanaen]

What happened?

When a query results in several CTEs, a computed column may be duplicated in a child CTE without its' column sources. This leads to Referenced column "..." not found as the column is not there anymore.

PRQL input

prql target:sql.postgres

from employees
derive `year` = s'EXTRACT(year from {`hire_date`})'
derive { `year_label` = f"Year {`year`}" }
derive { `city` = case [ this.`city` == "Calgary" => "A city", true => this.`city` ] }
derive { `city` = case [ this.`city` == "Edmonton" => "Another city", true => this.`city` ] }
group {`year`, `year_label`} (take 1)
select {this.`year_label`}

SQL output

WITH table_0 AS (
  SELECT
    CONCAT(
      'Year ',
      EXTRACT(
        year
        from
          hire_date
      )
    ) AS year_label,
    CASE
      WHEN city = 'Calgary' THEN 'A city'
      ELSE city
    END AS _expr_0,
    city
  FROM
    employees
)
SELECT
  DISTINCT ON (
    EXTRACT(
      year
      from
        hire_date
    ),
    year_label
  ) year_label
FROM
  table_0

Expected SQL output

WITH table_0 AS (
  SELECT
    CONCAT(
      'Year ',
      EXTRACT(
        year
        from
          hire_date
      )
    ) AS year_label,
    EXTRACT(
      year
      from
        hire_date
    ) as year_or_expr_1_or_something,
    CASE
      WHEN city = 'Calgary' THEN 'A city'
      ELSE city
    END AS _expr_0,
    city
  FROM
    employees
)
SELECT
  DISTINCT ON (
    year_or_expr_1_or_something,
    year_label
  ) year_label
FROM
  table_0

MVCE confirmation

• Minimal example
• New issue

Anything else?

Example is reproducible in the playground. Removing the last select is enough to get out of the CTE topology and therefore to not fail.