INSERT locking order

In an INSERT, are the rows guaranteed to be inserted in the order that
the source query returns its rows, for locking purposes?

For example, if have

  CREATE TABLE foo (
    foo_id INTEGER,
    PRIMARY KEY (foo_id)
  );

and I run two concurrent copies of

  INSERT INTO foo (SELECT n FROM ... ORDER BY n);

is there any guarantee that I'll get a unique constraint violation
rather than a deadlock?

-M-
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