distinct on doesn't fail without order by? why?

I was under impression that select distinct on (xx) ...
will fail if xx doesn't match the left most part of order by. i.e. it
requires order by xx, while allowing order by xx, something, else.

But it seems you can run the query with no order by clause at all.

is it intentional?

# select distinct on (x) x, y from test;
 x | y
---+---
 1 | 2
 2 | 3
(2 rows)

# select * from test;
 x | y
---+---
 1 | 2
 2 | 3
 1 | 4
(3 rows)

Best regards,

depesz

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