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4114. elements_view::iterator::operator* missing conditional noexcept specification

Section: 26.7.23.3 [range.elements.iterator] Status: New Submitter: Hewill Kang Opened: 2024-07-02 Last modified: 2024-08-02

Priority: 3

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Discussion:

views::elements, which can be seen as a specialization of views::transform, is used to transform the original tuple into its elements.

Since neither has a specialization for iter_move, the customization point is dispatched to the default implementation. However, unlike the latter, elements_view::iterator::operator* does not have a noexcept specification, which makes calls to iter_move never noexcept, which seems to be an oversight (demo):

#include <ranges>
#include <vector>

std::vector v{std::pair{1, "a"}, {2, "b"}};

auto r1 = v | std::views::keys;
auto i1 = r1.begin();
static_assert(noexcept(*i1));                        // failed
static_assert(noexcept(std::ranges::iter_move(i1))); // failed

auto get_key = [](auto& t) noexcept -> auto& { return std::get<0>(t); };
auto r2 = v | std::views::transform(get_key);
auto i2 = r2.begin();
static_assert(noexcept(*i2));
static_assert(noexcept(std::ranges::iter_move(i2)));

The proposed resolution is aligned with the strengthened implementation of MSVC-STL.

[2024-08-02; Reflector poll]

Set priority to 3 after reflector poll.

Proposed resolution:

This wording is relative to N4981.

1. Modify 26.7.23.3 [range.elements.iterator] as indicated:

   namespace std::ranges {
     template<input_range V, size_t N>
       requires view<V> && has-tuple-element<range_value_t<V>, N> &&
                has-tuple-element<remove_reference_t<range_reference_t<V>>, N> &&
                returnable-element<range_reference_t<V>, N>
     template<bool Const>
     class elements_view<V, N>::iterator {
       […]
   
       static constexpr decltype(auto) get-element(const iterator_t<Base>& i);     // exposition only
         noexcept(noexcept(std::get<N>(*i))) requires is_reference_v<range_reference_t<Base>> {
         return std::get<N>(*i);
       }
       
       static constexpr decltype(auto) get-element(const iterator_t<Base>& i)      // exposition only
         noexcept(noexcept(std::get<N>(*i)) &&
                  is_nothrow_move_constructible_v<tuple_element_t<N, range_reference_t<Base>>>) {
         using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>;
         return static_cast<E>(std::get<N>(*i));
       }
   
     public:
       […]
   
       constexpr decltype(auto) operator*() const
         noexcept(noexcept(get-element(current_)))
       { return get-element(current_); }
   
       […]
     };
   }
   

   […]

   static constexpr decltype(auto) get-element(const iterator_t<Base>& i);

   -3- Effects: Equivalent to:

   if constexpr (is_reference_v<range_reference_t<Base>>) {
     return std::get<N>(*i);
   } else {
     using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>;
     return static_cast<E>(std::get<N>(*i));
   }