66         struct is_view {  struct
67             static constexpr bool value = false;
94         struct is_view<sliced_view_t<Sequence, i...>> {  struct
95             static constexpr bool value = true;
117         struct is_view<transformed_view_t<Sequence, F>> {  struct
118             static constexpr bool value = true;
155         struct is_view<joined_view_t<Sequence1, Sequence2>> {  struct
156             static constexpr bool value = true;
176         struct is_view<single_view_t<T>> {  struct
177             static constexpr bool value = true;
194         struct is_view<empty_view_t> {  struct
195             static constexpr bool value = true;