Submitted by: @pavel-zotov

It seems that last build has a bug when query contains derived table:

Script:

show version;
select q.n, case when q.n=0 then 'zero' when q.n<>0 then 'NON-zero' else 'Hm!..' end what_is_n
from (select 0 N from RDB$DATABASE) q;

result in revision 58891:

ISQL Version: WI-V2.5.3.26661 Firebird 2.5
Server version:
Firebird/linux AMD64 (access method), version "LI-T3.0.0.30780 Firebird 3.0 Alpha 1"
Firebird/linux AMD64 (remote server), version "LI-T3.0.0.30780 Firebird 3.0 Alpha 1/tcp (oel64)/P12"
Firebird/x86/Windows NT (remote interface), version "WI-V2.5.3.26661 Firebird 2.5/tcp (csprog)/P12"
on disk structure version 12.0
SQL> select q.n, case when q.n=0 then 'zero' when q.n<>0 then 'NON-zero' else 'Hm!..' end what_is_n
CON> from (select 0 N from RDB$DATABASE) q;

       N WHAT\_IS\_N

============ =========
0 Hm!.. -- that seems strange... ;-)

result in some of previous versions:

ISQL Version: WI-V2.5.3.26661 Firebird 2.5
Server version:
Firebird/x86/Windows NT (access method), version "WI-T3.0.0.30663 Firebird 3.0 Alpha 1"
Firebird/x86/Windows NT (remote server), version "WI-T3.0.0.30663 Firebird 3.0 Alpha 1/tcp (CSMIRROR)/P12"
Firebird/x86/Windows NT (remote interface), version "WI-V2.5.3.26661 Firebird 2.5/tcp (csprog)/P12"
on disk structure version 12.0
SQL> select q.n, case when q.n=0 then 'zero' when q.n<>0 then 'NON-zero' else 'Hm!..' end what_is_n
CON> from (select 0 N from RDB$DATABASE) q;

       N WHAT\_IS\_N

============ =========
0 zero -- thats of course OK.

PS. Same 'Hm!..' result for other data types:

select q.n, case when q.n='a' then 'a' when q.n<>'a' then 'NON-a' else 'Hm!..' end what_is_n
from (select 'a' N from RDB$DATABASE) q;

select q.n, case when q.n=current_date then 'current_date' when q.n<>current_date then 'NON-current_date' else 'Hm!..' end what_is_n
from (select current_date N from RDB$DATABASE) q;