Submitted by: @cincuranet

Altering the function to same function changes the RDB$FIELD_SUB_TYPE to garbage value.

Here's repro:

$ .\isql.exe
Use CONNECT or CREATE DATABASE to specify a database
SQL> create database 'localhost:test' user 'SYSDBA' password 'masterkey';
SQL> create function test (i int) returns int as begin end;
SQL> commit;
SQL> select rdb$field_sub_type from rdb$function_arguments;

RDB$FIELD_SUB_TYPE

             0
             0

SQL> create or alter function test (i int) returns int as begin end;
SQL> commit;
SQL> select rdb$field_sub_type from rdb$function_arguments;

RDB$FIELD_SUB_TYPE

         17490
         17490

SQL>

Commits: f18eebb 48a9624