SELECT a/2, a/2 FROM test_missing_target GROUP BY a/2;
to be ambiguous ... which I think is correct behavior.

of the SELECT part of the statement is just like a plain SELECT.  All
INSERT-specific processing happens after the SELECT parsing is done.
This eliminates many problems, e.g. INSERT ... SELECT ... GROUP BY using
the wrong column labels.  Ensure that DEFAULT clauses are coerced to
the target column type, whether or not stored clause produces the right
type.  Substantial cleanup of parser's array support.

should be 'RangeTblEntry' ; explain.c had copied the erroneous code.