extra TARGETENTRY when group by function( col )?

Given:

create table t1 ( c1 timestamp without time zone, c2 int );

explain verbose select to_char ( c1, '<whatever>' ) from t1
group by to_char ( c1, '<whatever>' );

does any one know why the SEQSCAN node has TWO TARGETENTRYs?

Why do we project "c1" as well as "to_char( c1 . )" ?

---

For cases where we compute an aggregate, such as:

select min(c1), to_char(c1,.) from t1 group by to_char( c1,.)

it would make (some kind of) sense to project out "c1".

But can't the planner/optimizer distinguish these cases on the basis that
'min' is an aggregate function and 'to_char' is not?

The planner seems to be computing a minimal target list of columns (no
expressions) first. Guess I'm asking why it isn't replaced.