Non-selected columns for SELECT DISTINCT allowed in ORDER BY clause if ordered by expression

How to repeat:

ij> create table t (i integer, j integer);;
0 rows inserted/updated/deleted
ij> insert into t values (1,2),(1,3);
2 rows inserted/updated/deleted
ij> select distinct i from t order by j;
ERROR 42879: The ORDER BY clause may not contain column 'J', since the query specifies DISTINCT and that column does not appear in the query result.
ij> select distinct i from t order by j*2;
I
-----------
1
1

2 rows selected