Have we tried to treat CTE as SubQuery in planner?

Hi: 

Take the following example: 

insert into cte1  select i, i from generate_series(1, 1000000)i;
create index on cte1(a);

explain 

with cte1 as  (select * from cte1)

select * from c where a = 1;

It needs to do seq scan on the above format, however it is pretty

quick if we change the query to  

select * from (select * from cte1) c where a = 1;  

I know how we treat cte and subqueries differently currently,

I just don't know why we can't treat cte as a subquery, so lots of

subquery related technology can apply to it.  Do we have any 

discussion about this?  

Thanks