Implementing casting outside of the target struct

[David Zhang via Digitalmars-d-learn]

Hi,

I have two structs and want to cast between them. The cast is 
generalizable, so I want to keep it outside of the structs 
themselves (I'll be adding more later). How can I do this? I've 
tried the following so far:

    struct Event {
        EventID id;
        ubyte[32 - EventID.sizeof] payload;
    }

    struct WindowCreateEvent {
        enum id = EventID.windowCreate;

        version (Windows) {
            import core.sys.windows.windows;
            HWND windowHandle;
        }
    }

    T opCast(T, K)(auto ref K e) {
        import core.stdc.string: memcpy;

        auto result = Event(e.id);

        memcpy(result.payload.ptr, cast(void*) & e, K.sizeof);

        return result;
    }

The compiler seems to want to convert the two structs bit-by-bit. 
How can I make it use the module-scoped override? Thanks.