The sign of the number changes when bigint overflows

[Noremos]

String-to-Bigint cast works well

select cast('9.223372036854775e18' as bigint) from rdb$database; -- OK
                 CAST 
===================== 
  9223372036854775000 

select cast('9.223372036854775e19' as bigint) from rdb$database; -- OK
                 CAST 
===================== 
Statement failed, SQLSTATE = 22003
arithmetic exception, numeric overflow, or string truncation
-numeric value is out of range

But Double-to-BigInt cast changes the sign

SQL> select cast(9.223372036854775e18 as bigint) from rdb$database;  - Sign error

                 CAST 
===================== 
 -9223372036854775808 

SQL> select cast(9.223372036854775e19 as bigint) from rdb$database;  -- OK

                 CAST 
===================== 
Statement failed, SQLSTATE = 22003
arithmetic exception, numeric overflow, or string truncation
-numeric value is out of range