The type of == is always int, so the sizeof is sizeof(int), and the !! makes the result always be 1 (true). The entire purpose of the macro is to have the compiler warn if the types are incompatible.
Thanks! However, I still don't understand this completely. So the sizeof is there just to force evaluation of the comparison and because == can only be used among compatible types the compiler would warn? Why is the !! necessary then wouldn't sizeof(int) be enough as a "true" value?