Can't SELECT from (INSERT ... RETURNING)

I thought I could use the output of INSERT...RETURNING as a set of
tuples for a subquery,
but it seems it's not the case:

nb1=# select * from (insert into m(a) values(112) returning a);
ERROR: syntax error at or near "into"
LINE 1: select * from (insert into m(a) values(112) returni...
^

Is this a bug or it's not even supposed to work in theory?
Such a feature would be extremely useful to have.

P.S.
I know it's non-portable, but this is not a problem.

Thanks,

Nico