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3853. basic_const_iterator<volatile int*>::operator-> is ill-formed

Section: 25.5.3 [const.iterators] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

Currently, basic_const_iterator::operator-> constrains the value type of the underlying iterator to be only the cv-unqualified type of its reference type, which is true for raw pointers.

However, since it also explicitly specifies returning a pointer to a const value type, this will cause a hard error when the value type is actually volatile-qualified:

std::basic_const_iterator<volatile int*> it;
auto* p = it.operator->(); // invalid conversion from 'volatile int*' to 'const int*'

The proposed resolution changes the return type from const value_type* to const auto*, which makes it deduce the correct type in the above example, i.e. const volatile int*.

[2023-02-01; Reflector poll]

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

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.5.3 [const.iterators] as indicated:

   namespace std {
     template<class I>
       concept not-a-const-iterator = see below;
   
     template<input_iterator Iterator>
     class basic_const_iterator {
       Iterator current_ = Iterator();                             // exposition only
       using reference = iter_const_reference_t<Iterator>;         // exposition only
     
     public:
       […]
       constexpr reference operator*() const;
       constexpr const autovalue_type* operator->() const
          requires is_lvalue_reference_v<iter_reference_t<Iterator>> &&
                   same_as<remove_cvref_t<iter_reference_t<Iterator>>, value_type>;
       […]
     };
   }
   

   […]

   constexpr const autovalue_type* operator->() const
     requires is_lvalue_reference_v<iter_reference_t<Iterator>> &&
              same_as<remove_cvref_t<iter_reference_t<Iterator>>, value_type>;
   

   -7- Returns: If Iterator models contiguous_iterator, to_address(current_); otherwise, addressof(*current_).