I am not sure if this is a bug or it was intented to be so.
Using a constant defined by "enum" for calculating another constant's value before that first constant is mentioned in vc file does not cause any compiler error, while undefined constant seems to be treated as 0.
class DoomDefs : Object;
MYCONST = 100 + ANOTHER_CONST // ANOTHER_CONST is treated as 0 here
ANOTHER_CONST = 50