Case expression error with a select query in the condition

Question

Consider the two query below:
1. The first one use a case expression :
select case when 3 > (select 1 from dual)*2 then 1 else 0 end from dual
2. The equivalent with a decode expression:
select
decode(sign(3-(select 1 from dual)*2),1,1,0) from dual
The first query return an ORA-00905 error but not the second one. It seems that the arithmetic operation (select 1 from dual)*2 cause the error. The folowing query doesn't return this error:
select case when 3 > (select 2 from dual) then 1 else 0 end from dual
I run the test with Oracle 8.1.7.3 Enterprise Edition. Is it a known bug ?
Thomas

Answer 1

ora8174>select case when 3 > (select 1 from dual)*2 then 1 else 0 end from dual;
select case when 3 > (select 1 from dual)*2 then 1 else 0 end from dual
ERROR at line 1:
ORA-00905: missing keyword
ora8174>select case when 3 > cast((select 1 from dual)*2 as number) then 1 else 0 end from dual;
CASEWHEN3>CAST((SELECT1FROMDUAL)*2ASNUMBER)THEN1ELSE0END
                                                       1
1 row selected.
ora8174>select case when 3 > to_number((select 1 from dual)*2) then 1 else 0 end from dual;
CASEWHEN3>TO_NUMBER((SELECT1FROMDUAL)*2)THEN1ELSE0END
                                                    1
1 row selected.
ora8174>@connect
Connected.
ora9204>select case when 3 > (select 1 from dual)*2 then 1 else 0 end from dual;
CASEWHEN3>(SELECT1FROMDUAL)*2THEN1ELSE0END
                                         1
1 row selected.