typename A::element_type
typename A::data_handle_type
typename A::reference
typename A::offset_policy
a.access(p, i)
a.offset(p, i)
template<class OtherElementType>
constexpr default_accessor(default_accessor<OtherElementType>) noexcept {}
constexpr reference access(data_handle_type p, size_t i) const noexcept;
constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;