The invariant is less obvious, but you could still do this if you really wanted. The observation is that addition of two fixed-size nonnegative integers cannot overflow unless at least one is greater than half the range. So your `non_overflowing_addition` function would need take two inputs of type `NonNegativeLessThanHalfMaxValue`, where the constructor for the latter type enforces the invariant. Multiplication is similar, but with the square root of the range (and I suppose `NonNegativeLessThanSqrtMaxValue` could be a subtype of `NonNegativeLessThanHalfMaxValue` if you want to be fancy).