[range.transform] - C++17 → C++20

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+### Transform view <a id="range.transform">[[range.transform]]</a>
+#### Overview <a id="range.transform.overview">[[range.transform.overview]]</a>
+`transform_view` presents a `view` of an underlying sequence after
+applying a transformation function to each element.
+The name `views::transform` denotes a range adaptor object
+[[range.adaptor.object]]. Given subexpressions `E` and `F`, the
+expression `views::transform(E, F)` is expression-equivalent to
+`transform_view{E, F}`.
+[*Example 1*:
+``` cpp
+vector<int> is{ 0, 1, 2, 3, 4 };
+transform_view squares{is, [](int i) { return i * i; }};
+for (int i : squares)
+  cout << i << ' '; // prints: 0 1 4 9 16
+```
+— *end example*]
+#### Class template `transform_view` <a id="range.transform.view">[[range.transform.view]]</a>
+``` cpp
+namespace std::ranges {
+  template<input_range V, copy_constructible F>
+    requires view<V> && is_object_v<F> &&
+             regular_invocable<F&, range_reference_t<V>> &&
+             can-reference<invoke_result_t<F&, range_reference_t<V>>>
+  class transform_view : public view_interface<transform_view<V, F>> {
+  private:
+    // [range.transform.iterator], class template transform_view::iterator
+    template<bool> struct iterator;             // exposition only
+    // [range.transform.sentinel], class template transform_view::sentinel
+    template<bool> struct sentinel;             // exposition only
+    V base_ = V();                              // exposition only
+    semiregular-box<F> fun_;                    // exposition only
+  public:
+    transform_view() = default;
+    constexpr transform_view(V base, F fun);
+    constexpr V base() const& requires copy_constructible<V> { return base_; }
+    constexpr V base() && { return std::move(base_); }
+    constexpr iterator<false> begin();
+    constexpr iterator<true> begin() const
+      requires range<const V> &&
+               regular_invocable<const F&, range_reference_t<const V>>;
+    constexpr sentinel<false> end();
+    constexpr iterator<false> end() requires common_range<V>;
+    constexpr sentinel<true> end() const
+      requires range<const V> &&
+               regular_invocable<const F&, range_reference_t<const V>>;
+    constexpr iterator<true> end() const
+      requires common_range<const V> &&
+               regular_invocable<const F&, range_reference_t<const V>>;
+    constexpr auto size() requires sized_range<V> { return ranges::size(base_); }
+    constexpr auto size() const requires sized_range<const V>
+    { return ranges::size(base_); }
+  };
+  template<class R, class F>
+    transform_view(R&&, F) -> transform_view<views::all_t<R>, F>;
+}
+```
+``` cpp
+constexpr transform_view(V base, F fun);
+```
+*Effects:* Initializes *base\_* with `std::move(base)` and *fun\_* with
+`std::move(fun)`.
+``` cpp
+constexpr iterator<false> begin();
+```
+*Effects:* Equivalent to:
+``` cpp
+return iterator<false>{*this, ranges::begin(base_)};
+```
+``` cpp
+constexpr iterator<true> begin() const
+  requires range<const V> &&
+           regular_invocable<const F&, range_reference_t<const V>>;
+```
+*Effects:* Equivalent to:
+``` cpp
+return iterator<true>{*this, ranges::begin(base_)};
+```
+``` cpp
+constexpr sentinel<false> end();
+```
+*Effects:* Equivalent to:
+``` cpp
+return sentinel<false>{ranges::end(base_)};
+```
+``` cpp
+constexpr iterator<false> end() requires common_range<V>;
+```
+*Effects:* Equivalent to:
+``` cpp
+return iterator<false>{*this, ranges::end(base_)};
+```
+``` cpp
+constexpr sentinel<true> end() const
+  requires range<const V> &&
+           regular_invocable<const F&, range_reference_t<const V>>;
+```
+*Effects:* Equivalent to:
+``` cpp
+return sentinel<true>{ranges::end(base_)};
+```
+``` cpp
+constexpr iterator<true> end() const
+  requires common_range<const V> &&
+           regular_invocable<const F&, range_reference_t<const V>>;
+```
+*Effects:* Equivalent to:
+``` cpp
+return iterator<true>{*this, ranges::end(base_)};
+```
+#### Class template `transform_view::iterator` <a id="range.transform.iterator">[[range.transform.iterator]]</a>
+``` cpp
+namespace std::ranges {
+  template<input_range V, copy_constructible F>
+    requires view<V> && is_object_v<F> &&
+             regular_invocable<F&, range_reference_t<V>> &&
+             can-reference<invoke_result_t<F&, range_reference_t<V>>>
+  template<bool Const>
+  class transform_view<V, F>::iterator {
+  private:
+    using Parent =                              // exposition only
+      conditional_t<Const, const transform_view, transform_view>;
+    using Base   =                              // exposition only
+      conditional_t<Const, const V, V>;
+    iterator_t<Base> current_ =                 // exposition only
+      iterator_t<Base>();
+    Parent* parent_ = nullptr;                  // exposition only
+  public:
+    using iterator_concept  = see below;
+    using iterator_category = see below;
+    using value_type        =
+      remove_cvref_t<invoke_result_t<F&, range_reference_t<Base>>>;
+    using difference_type   = range_difference_t<Base>;
+    iterator() = default;
+    constexpr iterator(Parent& parent, 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 invoke(*parent_->fun_, *current_); }
+    constexpr iterator& operator++();
+    constexpr void operator++(int);
+    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 n)
+      requires random_access_range<Base>;
+    constexpr iterator& operator-=(difference_type n)
+      requires random_access_range<Base>;
+    constexpr decltype(auto) operator[](difference_type n) const
+      requires random_access_range<Base>
+    { return invoke(*parent_->fun_, 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& 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 auto operator<=>(const iterator& x, const iterator& y)
+      requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;
+    friend constexpr iterator operator+(iterator i, difference_type n)
+      requires random_access_range<Base>;
+    friend constexpr iterator operator+(difference_type n, iterator i)
+      requires random_access_range<Base>;
+    friend constexpr iterator operator-(iterator i, difference_type n)
+      requires random_access_range<Base>;
+    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
+      requires random_access_range<Base>;
+    friend constexpr decltype(auto) iter_move(const iterator& i)
+      noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_)))
+    {
+      if constexpr (is_lvalue_reference_v<decltype(*i)>)
+        return std::move(*i);
+      else
+        return *i;
+    }
+    friend constexpr void iter_swap(const iterator& x, const iterator& y)
+      noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)))
+      requires indirectly_swappable<iterator_t<Base>>;
+  };
+}
+```
+`iterator::iterator_concept` is defined as follows:
+- If `V` models `random_access_range`, then `iterator_concept` denotes
+  `random_access_iterator_tag`.
+- Otherwise, if `V` models `bidirectional_range`, then
+  `iterator_concept` denotes `bidirectional_iterator_tag`.
+- Otherwise, if `V` models `forward_range`, then `iterator_concept`
+  denotes `forward_iterator_tag`.
+- Otherwise, `iterator_concept` denotes `input_iterator_tag`.
+`iterator::iterator_category` is defined as follows: Let `C` denote the
+type `iterator_traits<iterator_t<Base>>::iterator_category`.
+- If
+  `is_lvalue_reference_v<invoke_result_t<F&, range_reference_t<Base>>>`
+  is `true`, then
+  - if `C` models `derived_from<contiguous_iterator_tag>`,
+    `iterator_category` denotes `random_access_iterator_tag`;
+  - otherwise, `iterator_category` denotes `C`.
+- Otherwise, `iterator_category` denotes `input_iterator_tag`.
+``` cpp
+constexpr iterator(Parent& parent, iterator_t<Base> current);
+```
+*Effects:* Initializes *current\_* with `std::move(current)` and
+*parent\_* with `addressof(parent)`.
+``` cpp
+constexpr iterator(iterator<!Const> i)
+  requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
+```
+*Effects:* Initializes *current\_* with `std::move(i.`*`current_`*`)`
+and *parent\_* with `i.`*`parent_`*.
+``` 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);
+```
+*Effects:* Equivalent to `++current_`.
+``` cpp
+constexpr iterator operator++(int) requires forward_range<Base>;
+```
+*Effects:* Equivalent to:
+``` cpp
+auto tmp = *this;
+++*this;
+return tmp;
+```
+``` 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 tmp = *this;
+--*this;
+return tmp;
+```
+``` 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<iterator_t<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+(iterator i, difference_type n)
+  requires random_access_range<Base>;
+friend constexpr iterator operator+(difference_type n, iterator i)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to:
+`return iterator{*i.`*`parent_`*`, i.`*`current_`*` + n};`
+``` cpp
+friend constexpr iterator operator-(iterator i, difference_type n)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to:
+`return iterator{*i.`*`parent_`*`, i.`*`current_`*` - n};`
+``` cpp
+friend constexpr difference_type operator-(const iterator& x, const iterator& y)
+  requires random_access_range<Base>;
+```
+*Effects:* Equivalent to: `return x.`*`current_`*` - y.`*`current_`*`;`
+``` cpp
+friend constexpr void iter_swap(const iterator& x, const iterator& y)
+  noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)))
+  requires indirectly_swappable<iterator_t<Base>>;
+```
+*Effects:* Equivalent to
+`ranges::iter_swap(x.`*`current_`*`, y.`*`current_`*`)`.
+#### Class template `transform_view::sentinel` <a id="range.transform.sentinel">[[range.transform.sentinel]]</a>
+``` cpp
+namespace std::ranges {
+  template<input_range V, copy_constructible F>
+    requires view<V> && is_object_v<F> &&
+             regular_invocable<F&, range_reference_t<V>> &&
+             can-reference<invoke_result_t<F&, range_reference_t<V>>>
+  template<bool Const>
+  class transform_view<V, F>::sentinel {
+  private:
+    using Parent =                                      // exposition only
+      conditional_t<Const, const transform_view, transform_view>;
+    using Base = conditional_t<Const, const V, V>;      // exposition only
+    sentinel_t<Base> end_ = sentinel_t<Base>();         // exposition only
+  public:
+    sentinel() = default;
+    constexpr explicit sentinel(sentinel_t<Base> end);
+    constexpr sentinel(sentinel<!Const> i)
+      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
+    constexpr sentinel_t<Base> base() const;
+    friend constexpr bool operator==(const iterator<Const>& x, const sentinel& y);
+    friend constexpr range_difference_t<Base>
+      operator-(const iterator<Const>& x, const sentinel& y)
+        requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
+    friend constexpr range_difference_t<Base>
+      operator-(const sentinel& y, const iterator<Const>& x)
+        requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
+  };
+}
+```
+``` cpp
+constexpr explicit sentinel(sentinel_t<Base> end);
+```
+*Effects:* Initializes *end\_* with `end`.
+``` cpp
+constexpr sentinel(sentinel<!Const> i)
+  requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
+```
+*Effects:* Initializes *end\_* with `std::move(i.`*`end_`*`)`.
+``` cpp
+constexpr sentinel_t<Base> base() const;
+```
+*Effects:* Equivalent to: `return `*`end_`*`;`
+``` cpp
+friend constexpr bool operator==(const iterator<Const>& x, const sentinel& y);
+```
+*Effects:* Equivalent to: `return x.`*`current_`*` == y.`*`end_`*`;`
+``` cpp
+friend constexpr range_difference_t<Base>
+  operator-(const iterator<Const>& x, const sentinel& y)
+    requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
+```
+*Effects:* Equivalent to: `return x.`*`current_`*` - y.`*`end_`*`;`
+``` cpp
+friend constexpr range_difference_t<Base>
+  operator-(const sentinel& y, const iterator<Const>& x)
+    requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
+```
+*Effects:* Equivalent to: `return y.`*`end_`*` - x.`*`current_`*`;`

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