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3449. take_view and take_while_view's sentinel<false> not comparable with their const iterator

Section: 26.7.10.3 [range.take.sentinel], 26.7.11.3 [range.take.while.sentinel] Status: WP Submitter: Tim Song Opened: 2020-06-06 Last modified: 2020-11-09

Priority: 1

View all other issues in [range.take.sentinel].

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Discussion:

During reflector discussion it was noted that the issue observed in LWG 3448 applies to take_view::sentinel and take_while_view::sentinel as well.

[2020-06-15 Tim corrects a typo in the PR]

[2020-10-02; Priority set to 1 in telecon]

Should be considered together with 3406 and 3448.

[2020-10-02; Status to Tentatively Ready after five positive votes on the reflector]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861. It assumes the maybe-const helper introduced by the P/R of LWG 3448.

1. Modify 26.7.10.3 [range.take.sentinel] as indicated:

   [Drafting note: Unlike LWG 3448, these operators can't be easily templatized, so the approach taken here is to add an overload for the opposite constness instead. However, because const V may not even be a range (and therefore iterator_t<const V> may not be well-formed), we need to defer instantiating the signature of the new overload until the point of use. ]

   namespace std::ranges {
     template<view V>
     template<bool Const>
     class take_view<V>::sentinel {
     private:
       using Base = conditional_t<Const, const V, V>;  // exposition only
       template<bool OtherConst>
       using CI = counted_iterator<iterator_t<Basemaybe-const<OtherConst, V>>>;  // exposition only
   
       […]
   
       constexpr sentinel_t<Base> base() const;
   
       friend constexpr bool operator==(const CI<Const>& y, const sentinel& x);
       
       template<bool OtherConst = !Const>
           requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
       friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);
       
     };
   }
   

   […]

   friend constexpr bool operator==(const CI<Const>& y, const sentinel& x);
   
   template<bool OtherConst = !Const>
       requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);
   
   

   -4- Effects: Equivalent to: return y.count() == 0 || y.base() == x.end_;

2. Modify 26.7.11.3 [range.take.while.sentinel] as indicated:

   namespace std::ranges {
       template<view V, class Pred>
       requires input_range<V> && is_object_v<Pred> &&
                   indirect_unary_predicate<const Pred, iterator_t<V>>
       template<bool Const>
       class take_while_view<V>::sentinel {
         […]
   
         constexpr sentinel_t<Base> base() const { return end_; }
   
         friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y);
         
         template<bool OtherConst = !Const>
             requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
         friend constexpr bool operator==(const iterator_t<maybe-const<OtherConst, V>>& x, const sentinel& y);
         
       };
   }
   

   […]

   friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y);
   
   template<bool OtherConst = !Const>
       requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   friend constexpr bool operator==(const iterator_t<maybe-const<OtherConst, V>>& x, const sentinel& y);
   
   

   -3- Effects: Equivalent to: return y.end_ == x || !invoke(*y.pred_, *x);