using const_iterator = implementation-defined;
A type that meets the requirements of a constant random access iterator ([random.access.iterators]) and of a contiguous iterator ([iterator.requirements.general]) whose value_type is the template parameter charT.
For a basic_string_view str, any operation that invalidates a pointer in the range [str.data(), str.data() + str.size()) invalidates pointers, iterators, and references returned from str's methods.
All requirements on container iterators ([container.requirements]) apply to basic_string_view::const_iterator as well.
constexpr const_iterator begin() const noexcept;
constexpr const_iterator cbegin() const noexcept;
constexpr const_iterator end() const noexcept;
constexpr const_iterator cend() const noexcept;
constexpr const_reverse_iterator rbegin() const noexcept;
constexpr const_reverse_iterator crbegin() const noexcept;
constexpr const_reverse_iterator rend() const noexcept;
constexpr const_reverse_iterator crend() const noexcept;