Numeric

NaN equivalent for integer data types

NaN is defined only for floating point numbers. In a U32 all the possible values are actual numbers and there is no place left for a special value that says there is nothing there. If you need an "empty" value in an integer data type you have to pick a value that never appears in your particular application (if there really is one) to serve as a flag. In other words there is no "empty value" in integer data types.