Test case:

create table t1 (col int primary key);
create procedure p1 returns (ret int) as begin ret = 0; suspend; end;
create procedure p2 (prm int) returns (ret int) as begin ret = prm; suspend; end;
commit;
insert into t1 (col) values (0);
commit;

Below comments indicate the expected result:

select * from t1 where col = 0;
-- index
select * from t1 where col = col;
-- natural
select * from t1 where col = ( select 0 from rdb$database );
-- index
select * from t1 where col = ( select col from rdb$database );
-- natural
select * from t1 where col = ( select 0 from p1 );
-- index (currently natural)
select * from t1 where col = ( select ret from p1 );
-- index (currently natural)
select * from t1 where col = ( select col from p1 );
-- natural
select * from t1 where col = ( select 0 from p2(0) );
-- index (currently natural)
select * from t1 where col = ( select ret from p2(0) );
-- index (currently natural)
select * from t1 where col = ( select col from p2(0) );
-- natural
select * from t1 where col = ( select 0 from p2(col) );
-- natural
select * from t1 where col = ( select ret from p2(col) );
-- natural
select * from t1 where col = ( select col from p2(col) );
-- natural