Issue 3574: common_iterator should be completely constexpr-able

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3574. `common_iterator` should be completely `constexpr`-able

Section: 24.5.5.1 [common.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2021-07-21 Last modified: 2023-11-22

Priority: 3

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Discussion:

After P2231R1, variant becomes completely constexpr-able, which means that common_iterator can also be completely constexpr-able.

[2021-08-20; Reflector poll]

Set priority to 3 after reflector poll.

[2021-08-23; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

Modify 24.5.5.1 [common.iterator], class template common_iterator synopsis, as indicated:

namespace std {
  template<input_or_output_iterator I, sentinel_for<I> S>
    requires (!same_as<I, S> && copyable<I>)
  class common_iterator {
  public:
    constexpr common_iterator() requires default_initializable<I> = default;
    constexpr common_iterator(I i);
    constexpr common_iterator(S s);
    template<class I2, class S2>
      requires convertible_to<const I2&, I> && convertible_to<const S2&, S>
        constexpr common_iterator(const common_iterator<I2, S2>& x);
        
    template<class I2, class S2>
      requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
               assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
      constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);

    constexpr decltype(auto) operator*();
    constexpr decltype(auto) operator*() const
      requires dereferenceable<const I>;
    constexpr decltype(auto) operator->() const
      requires see below;
 
    constexpr common_iterator& operator++();
    constexpr decltype(auto) operator++(int);

    template<class I2, sentinel_for<I> S2>
      requires sentinel_for<S, I2>
    friend constexpr bool operator==(
      const common_iterator& x, const common_iterator<I2, S2>& y);
    template<class I2, sentinel_for<I> S2>
      requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
    friend constexpr bool operator==(
      const common_iterator& x, const common_iterator<I2, S2>& y);

    template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
      requires sized_sentinel_for<S, I2>
    friend constexpr iter_difference_t<I2> operator-(
      const common_iterator& x, const common_iterator<I2, S2>& y);

    friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i)
      noexcept(noexcept(ranges::iter_move(declval<const I&>())))
        requires input_iterator<I>;
    template<indirectly_swappable<I> I2, class S2>
      friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
        noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));

  private:
    variant<I, S> v_; // exposition only
  };
  […]
}

Modify 24.5.5.3 [common.iter.const] as indicated:

template<class I2, class S2>
  requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
           assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
    constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);

-5- Preconditions: x.v_.valueless_by_exception() is false.
[…]

Modify 24.5.5.4 [common.iter.access] as indicated:

constexpr decltype(auto) operator*();
constexpr decltype(auto) operator*() const
  requires dereferenceable<const I>;
-1- Preconditions: holds_alternative<I>(v_) is true.
[…]
constexpr decltype(auto) operator->() const
  requires see below;
-3- The expression in the requires-clause is equivalent to:
[…]

Modify 24.5.5.5 [common.iter.nav] as indicated:
```
constexpr common_iterator& operator++();
```
-1- Preconditions: holds_alternative<I>(v_) is true.
[…]
```
constexpr decltype(auto) operator++(int);
```
-4- Preconditions: holds_alternative<I>(v_) is true.
[…]

Modify 24.5.5.6 [common.iter.cmp] as indicated:

template<class I2, sentinel_for<I> S2>
  requires sentinel_for<S, I2>
friend constexpr bool operator==(
  const common_iterator& x, const common_iterator<I2, S2>& y);

-1- Preconditions: […]

template<class I2, sentinel_for<I> S2>
  requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
friend constexpr bool operator==(
  const common_iterator& x, const common_iterator<I2, S2>& y);

-3- Preconditions: […]

template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
  requires sized_sentinel_for<S, I2>
friend constexpr iter_difference_t<I2> operator-(
  const common_iterator& x, const common_iterator<I2, S2>& y);

-5- Preconditions: […]

Modify 24.5.5.7 [common.iter.cust] as indicated:

friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i)
  noexcept(noexcept(ranges::iter_move(declval<const I&>())))
    requires input_iterator<I>;

-1- Preconditions: […]

template<indirectly_swappable<I> I2, class S2>
  friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
    noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));

-3- Preconditions: […]

3574. common_iterator should be completely constexpr-able

3574. `common_iterator` should be completely `constexpr`-able