[range.elements.iterator] - C++17 → C++20

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+#### Class template `elements_view::iterator` <a id="range.elements.iterator">[[range.elements.iterator]]</a>
+``` cpp
+namespace std::ranges {
+  template<input_range V, size_t N>
+    requires view<V> && has-tuple-element<range_value_t<V>, N> &&
+             has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
+  template<bool Const>
+  class elements_view<V, N>::iterator {                 // exposition only
+    using Base = conditional_t<Const, const V, V>;      // exposition only
+    iterator_t<Base> current_ = iterator_t<Base>();
+  public:
+    using iterator_category = typename iterator_traits<iterator_t<Base>>::iterator_category;
+    using value_type = remove_cvref_t<tuple_element_t<N, range_value_t<Base>>>;
+    using difference_type = range_difference_t<Base>;
+    iterator() = default;
+    constexpr explicit iterator(iterator_t<Base> current);
+    constexpr iterator(iterator<!Const> i)
+      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
+    constexpr iterator_t<Base> base() const&
+      requires copyable<iterator_t<Base>>;
+    constexpr iterator_t<Base> base() &&;
+    constexpr decltype(auto) operator*() const
+    { return get<N>(*current_); }
+    constexpr iterator& operator++();
+    constexpr void operator++(int) requires (!forward_range<Base>);
+    constexpr iterator operator++(int) requires forward_range<Base>;
+    constexpr iterator& operator--() requires bidirectional_range<Base>;
+    constexpr iterator operator--(int) requires bidirectional_range<Base>;
+    constexpr iterator& operator+=(difference_type x)
+      requires random_access_range<Base>;
+    constexpr iterator& operator-=(difference_type x)
+      requires random_access_range<Base>;
+    constexpr decltype(auto) operator[](difference_type n) const
+      requires random_access_range<Base>
+    { return get<N>(*(current_ + n)); }
+    friend constexpr bool operator==(const iterator& x, const iterator& y)
+      requires equality_comparable<iterator_t<Base>>;
+    friend constexpr bool operator<(const iterator& x, const iterator& y)
+      requires random_access_range<Base>;
+    friend constexpr bool operator>(const iterator& x, const iterator& y)
+      requires random_access_range<Base>;
+    friend constexpr bool operator<=(const iterator& y, const iterator& y)
+      requires random_access_range<Base>;
+    friend constexpr bool operator>=(const iterator& x, const iterator& y)
+      requires random_access_range<Base>;
+    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
+      requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;
+    friend constexpr iterator operator+(const iterator& x, difference_type y)
+      requires random_access_range<Base>;
+    friend constexpr iterator operator+(difference_type x, const iterator& y)
+      requires random_access_range<Base>;
+    friend constexpr iterator operator-(const iterator& x, difference_type y)
+      requires random_access_range<Base>;
+    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
+      requires random_access_range<Base>;
+  };
+}
+```
+``` cpp
+constexpr explicit iterator(iterator_t<Base> current);
+```
+*Effects:* Initializes *current\_* with `std::move(current)`.
+``` cpp
+constexpr iterator(iterator<!Const> i)
+  requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
+```
+*Effects:* Initializes *current\_* with `std::move(i.`*`current_`*`)`.
+``` cpp
+constexpr iterator_t<Base> base() const&
+  requires copyable<iterator_t<Base>>;
+```
+*Effects:* Equivalent to: `return `*`current_`*`;`
+``` cpp
+constexpr iterator_t<Base> base() &&;
+```
+*Effects:* Equivalent to: `return std::move(`*`current_`*`);`
+``` cpp
+constexpr iterator& operator++();
+```
+*Effects:* Equivalent to:
+``` cpp
+++current_;
+return *this;
+```
+``` cpp
+constexpr void operator++(int) requires (!forward_range<Base>);
+```
+*Effects:* Equivalent to: `++`*`current_`*.
+``` cpp
+constexpr iterator operator++(int) requires forward_range<Base>;
+```
+*Effects:* Equivalent to:
+``` cpp
+auto temp = *this;
+++current_;
+return temp;
+```
+``` cpp
+constexpr iterator& operator--() requires bidirectional_range<Base>;
+```
+*Effects:* Equivalent to:
+``` cpp
+--current_;
+return *this;
+```
+``` cpp
+constexpr iterator operator--(int) requires bidirectional_range<Base>;
+```
+*Effects:* Equivalent to:
+``` cpp
+auto temp = *this;
+--current_;
+return temp;
+```
+``` cpp
+constexpr iterator& operator+=(difference_type n);
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to:
+``` cpp
+current_ += n;
+return *this;
+```
+``` cpp
+constexpr iterator& operator-=(difference_type n)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to:
+``` cpp
+current_ -= n;
+return *this;
+```
+``` cpp
+friend constexpr bool operator==(const iterator& x, const iterator& y)
+  requires equality_comparable<Base>;
+```
+*Effects:* Equivalent to: `return x.`*`current_`*` == y.`*`current_`*`;`
+``` cpp
+friend constexpr bool operator<(const iterator& x, const iterator& y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return x.`*`current_`*` < y.`*`current_`*`;`
+``` cpp
+friend constexpr bool operator>(const iterator& x, const iterator& y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return y < x;`
+``` cpp
+friend constexpr bool operator<=(const iterator& x, const iterator& y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return !(y < x);`
+``` cpp
+friend constexpr bool operator>=(const iterator& x, const iterator& y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return !(x < y);`
+``` cpp
+friend constexpr auto operator<=>(const iterator& x, const iterator& y)
+  requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;
+```
+*Effects:* Equivalent to:
+`return x.`*`current_`*` <=> y.`*`current_`*`;`
+``` cpp
+friend constexpr iterator operator+(const iterator& x, difference_type y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return iterator{x} += y;`
+``` cpp
+friend constexpr iterator operator+(difference_type x, const iterator& y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return y + x;`
+``` cpp
+constexpr iterator operator-(const iterator& x, difference_type y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return iterator{x} -= y;`
+``` cpp
+constexpr difference_type operator-(const iterator& x, const iterator& y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return x.`*`current_`*` - y.`*`current_`*`;`

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