+ assert sub isa MClassType else print_error "{sub} <? {sup}" # It is the only remaining type
+
+ # Handle sup-type when the sub-type is class-based (other cases must have be identified before).
+ if sup isa MFormalType or sup isa MNullType or sup isa MBottomType or sup isa MErrorType then
+ # These types are not super-types of Class-based types.