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3762. generator::iterator::operator== should pass by reference

Section: 26.8.6 [coro.generator.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

Currently, generator::iterator::operator== passes iterator by value. Given that iterator is a move-only type, this makes it impossible for generator to model range, since default_sentinel cannot be compared to the iterator of const lvalue and is not eligible to be its sentinel.

[2022-09-23; Reflector poll]

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

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 26.8.6 [coro.generator.iterator] as indicated:

   namespace std {
     template<class Ref, class V, class Allocator>
     class generator<Ref, V, Allocator>::iterator {
     public:
       using value_type = value;
       using difference_type = ptrdiff_t;
   
       iterator(iterator&& other) noexcept;
       iterator& operator=(iterator&& other) noexcept;
       
       reference operator*() const noexcept(is_nothrow_copy_constructible_v<reference>);
       iterator& operator++();
       void operator++(int);
   
       friend bool operator==(const iterator& i, default_sentinel_t);
   
     private:
       coroutine_handle<promise_type> coroutine_; // exposition only
     };
   }
   

   […]

   friend bool operator==(const iterator& i, default_sentinel_t);
   

   -10- Effects: Equivalent to: return i.coroutine_.done();