Unable to resolve column alias in PostgreSQL DISTINCT ON expression.

You can use column aliases within `DISTINCT ON` expressions. But it seems to be unable to resolve.

A more concise example:

These are all completely valid statements.

Here are the statements:

CREATE TABLE some_table (
    PRIMARY KEY (id, n),
    id text,
    n numeric
);

INSERT INTO some_table
SELECT 'foo', random()
FROM generate_series(1, 10);

SELECT DISTINCT ON (id, expr)
    id,
    sum(n) OVER (PARTITION BY id) AS expr
FROM some_table;

SELECT DISTINCT ON (n) 1 AS n;

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