[incrementable.traits]

To implement algorithms only in terms of incrementable types, it is often necessary to determine the difference type that corresponds to a particular incrementable type.

Accordingly, it is required that if WI is the name of a type that models the weakly_incrementable concept ([iterator.concept.winc]), the type iter_difference_t<WI> be defined as the incrementable type's difference type.

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namespace std { template<class> struct incrementable_traits { }; template<class T> requires is_object_v<T> struct incrementable_traits<T*> { using difference_type = ptrdiff_t; }; template<class I> struct incrementable_traits<const I> : incrementable_traits<I> { }; template<class T> requires requires { typename T::difference_type; } struct incrementable_traits<T> { using difference_type = T::difference_type; }; template<class T> requires (!requires { typename T::difference_type; } && requires(const T& a, const T& b) { a - b } -> integral; }) struct incrementable_traits<T> { using difference_type = make_signed_t<decltype(declval<T>() - declval<T>()>; }; template<class T> using iter_difference_t = see below; }

Let

{R I be remove_cvref_t < I > .}_{I be remove_cvref_t < I > .}

The type iter_difference_t<I> denotes

(2.1)
incrementable_traits< ${R I > :: difference_type if iterator_traits < {R I > names a specialization generated from the primary template, and}_{I > names a specialization generated from the primary template, and}}_{I > :: difference_type if iterator_traits < {R I > names a specialization generated from the primary template, and}_{I > names a specialization generated from the primary template, and}}$
(2.2)
iterator_traits< ${R I > :: difference_type otherwise .}_{I > :: difference_type otherwise .}$

Users may specialize incrementable_traits on program-defined types.