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4116. enumerate_view::iterator and cartesian_product_view::iterator should not always provide iterator_category

Section: 25.7.24.3 [range.enumerate.iterator], 25.7.33.3 [range.cartesian.iterator] Status: New Submitter: Hewill Kang Opened: 2024-07-07 Last modified: 2025-10-21

Priority: 3

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Discussion:

These two iterators do not support *r++ for non-forward iterators, so we should not provide iterator_category as they are not C++17 iterators.

[2025-10-21; Reflector poll.]

Set priority to 3 after reflector poll.

Six votes for Tentatively Ready, but not unanimous.

Proposed resolution:

This wording is relative to N4981.

1. Modify 25.7.24.3 [range.enumerate.iterator] as indicated:

   namespace std::ranges {
     template<view V>
       requires range-with-movable-references<V>
     template<bool Const>
     class enumerate_view<V>::iterator {
       using Base = maybe-const<Const, V>;                         // exposition only
   
     public:
       using iterator_category = input_iterator_tag;      // present only if Base
                                                          // models forward_range
       using iterator_concept = see below;
       […]
     };
   }
   

2. Modify 25.7.33.3 [range.cartesian.iterator] as indicated:

   namespace std::ranges {
     template<input_range First, forward_range... Vs>
       requires (view<First> && ... && view<Vs>)
     template<bool Const>
     class cartesian_product_view<First, Vs...>::iterator {
     public:
       using iterator_category = input_iterator_tag;      // present only if maybe-const<Const, First>
                                                          // models forward_range
       using iterator_concept  = see below;
       […]
     };
   }