create or replace function f1(p1 varchar2, p2 varchar2)
return varchar2
as
begin
  return p1||' '||p2;
end;
/


column c1 format a10
column c2 format a10
with q as (
  select f1(p2=>'b', p1 => 'a') as c1 from dual
)
select c1, c1 as c2
from q;

SQL> column c1 format a10
SQL> column c2 format a10
SQL> with q as (
  2    select f1(p2=>'b', p1 => 'a') as c1 from dual
  3  )
  4  select c1, c1 as c2
  5  from q;

C1         C2
---------- ----------
b a        a b

SQL> set autotrace traceonly explain
SQL> with q as (
  2    select f1(p2=>'b', p1 => 'a') as c1 from dual
  3  )
  4  select c1, c1 as c2
  5  from q
  6  where c1='a';

Execution Plan
----------------------------------------------------------
Plan hash value: 4034615273

-----------------------------------------------------------------
| Id  | Operation        | Name | Rows  | Cost (%CPU)| Time     |
-----------------------------------------------------------------
|   0 | SELECT STATEMENT |      |     1 |     2   (0)| 00:00:01 |
|*  1 |  FILTER          |      |       |            |          |
|   2 |   FAST DUAL      |      |     1 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("F1"("P1"=>'b',"P2"=>'a')='a')

SQL>