There are only 4 signatures that Main can have: 1) public static def Main() as void 2) public static def Main() as int 3) public static def Main(args as (string)) as void 4) public static def Main(args as (string)) as int Boo compiles: public static def Main(*args) as <any type>: pass public static def Main(args as (int)) as <any type>: pass Upon runtime, the result is: Unhandled Exception: System.MethodAccessException: Main method for type 80131510 has invalid signature.