std::layout_stride::mapping::layout-mapping-alike

template<class T>
  constexpr bool is-extents = false;                              // exposition only
template<class IndexType, size_t... Args>
  constexpr bool is-extents<extents<IndexType, Args...>> = true;  // exposition only

template<class M>
concept layout-mapping-alike = requires {  // exposition only
  requires is-extents<typename M::extents_type>;
  { M::is_always_strided() } -> same_as<bool>;
  { M::is_always_exhaustive() } -> same_as<bool>;
  { M::is_always_unique() } -> same_as<bool>;
  bool_constant<M::is_always_strided()>::value;
  bool_constant<M::is_always_exhaustive()>::value;
  bool_constant<M::is_always_unique()>::value;
};