std::experimental::ranges::projected - cppreference.com

From cppreference.com

template< Readable I, IndirectRegularUnaryInvocable<I> Proj >
struct projected {
    using value_type =
        std::remove_cv_t<std::remove_reference_t<ranges::indirect_result_of_t<Proj&(I)>>>;
    ranges::indirect_result_of_t<Proj&(I)> operator*() const;
};

template< WeaklyIncrementable I, class Proj >
struct difference_type<projected<I, Proj>> {
    using type = ranges::difference_type_t<I>;
};

(ranges TS)

The class template projected bundles a Readable type I and a function Proj into a new Readable type whose reference type is the result of applying Proj to the reference type of I. It exists solely to ease constraint specification, and so its operator*() may not be actually defined.