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3387. §[range.reverse.view] reverse_view<V> unintentionally requires range<const V>

Section: 26.7.20.2 [range.reverse.view] Status: C++20 Submitter: Patrick Palka Opened: 2020-02-04 Last modified: 2021-02-25

Priority: 0

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Discussion:

reverse_view<V> requires bidirectional_range<V>, but not range<const V>, which means that iterator_t<const V> might be an invalid type. The return types of the begin() const and end() const overloads make use of iterator_t<const V> in a non-SFINAE context, which means that instantiating reverse_view<X> is ill-formed unless range<const X> is satisfied.

Code like x | views::filter(p) | views::reverse fails to compile because const filter_view<…> does not model range, so iterator_t<const filter_view<…>> is invalid.

Either range<const V> needs to be in the class' requires-clause, or the return types of the const-qualified begin() and end() need to delay use of iterator_t<const V> until range<const V> is known to be satisfied.

Giving these overloads an auto return type means the type is determined when the member is called. The constraint common_range<const V> appropriately restricts the selection of these overloads, so they can only be called when the type is valid. This is what cmcstl2 does. range-v3 makes the begin() const and end() const members into function templates, so that they are SFINAE contexts.

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 26.7.20.2 [range.reverse.view] as indicated:

   namespace std::ranges {
     template<view V>
       requires bidirectional_range<V>
     class reverse_view : public view_interface<reverse_view<V>> {
       […]
       constexpr reverse_iterator<iterator_t<V>> begin();
       constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
       constexpr reverse_iterator<iterator_t<const V>>auto begin() const
         requires common_range<const V>;  
       
       constexpr reverse_iterator<iterator_t<V>> end();
       constexpr reverse_iterator<iterator_t<const V>>auto end() const
         requires common_range<const V>;  
       […]
     };
     […]
   }
   

   […]

   constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
   constexpr reverse_iterator<iterator_t<const V>>auto begin() const
     requires common_range<const V>;
   

   -5- Effects: Equivalent to: return make_reverse_iterator(ranges::end(base_));

   constexpr reverse_iterator<iterator_t<V>> end();
   constexpr reverse_iterator<iterator_t<const V>>auto end() const
     requires common_range<const V>;
   

   -6- Effects: Equivalent to: return make_reverse_iterator(ranges::begin(base_));