Several of the random number distributions use make_[un]signed internally. However, this trait is not intended as a customization point for UDT's - it's only supposed to be instantiated for integer or enum types - and yes that's a deliberate decision that's now enshrined in the standard ;-) Unfortunately this means that these distributions can't be instantiated on UDT's - big number types for example. Since it seems that these traits are only used to obtain a type that is large enough to store a difference between the endpoints of a range, would it be possible for these to change to use something like: namespace boost{ namespace random{ template <class T> struct range_type : public make_signed<T> { }; }} Then range_type could be [partially-] specialized for UDT's. Note that for some types, there might simply not be a corresponding unsigned type at all, but I'm assuming that doesn't foul up the logic, as long as the type is wide enough? Comments? Thanks, John.