Proposed resolution:

This wording is relative to N4892.

1. Modify [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
     };
     […]
   }
   

2. Modify [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.

   […]

3. Modify [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:

   […]

4. Modify [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.

   […]

5. Modify [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: […]

6. Modify [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: […]

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

[ 2021-08-23; Reflector poll ]

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

[ 2021-08-20; Reflector poll ]

Set priority to 3 after reflector poll.

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