dist_id is undocumented

[Scott Baden]

What is the purpose of dist_id() ?

template < typename T> struct dist_id <T >;

C++ Concepts: PODType, EqualityComparable, LessThanComparable, hashable

[Dan Bonachea]

See spec sections 12.2 and 12.3

Bottom-line, most use cases never need the type, but it's available for more advanced use.