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Section: 26.7.14.2 [range.join.view], 26.7.15.2 [range.join.with.view] Status: Resolved Submitter: Hewill Kang Opened: 2022-05-17 Last modified: 2023-03-23
Priority: 3
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Discussion:
This is a follow-up of LWG 3599.
Currently, the const version of join_view::begin has the following constraints
constexpr auto begin() const
requires input_range<const V> &&
is_reference_v<range_reference_t<const V>>
{ return iterator<true>{*this, ranges::begin(base_)}; }
which only requires InnerRng to be a reference type, but in some cases, InnerRng may not be a range. Consider
#include <ranges>
int main() {
auto r = std::views::iota(0, 5)
| std::views::split(1);
auto s = std::views::single(r);
auto j = s | std::views::join;
auto f = j.front();
}
The reference type of single_view is const split_view&, which only has the non-const version of begin, which will cause view_interface's const front to be incorrectly instantiated, making r.front() unnecessarily ill-formed.
We should add this check for join_view's const begin, as well as join_with_view.
[2022-06-21; Reflector poll]
Set priority to 3 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 26.7.14.2 [range.join.view], class template join_view synopsis, as indicated:
namespace std::ranges { […] template<input_range V> requires view<V> && input_range<range_reference_t<V>> class join_view : public view_interface<join_view<V>> { private: […] public: […] constexpr auto begin() { […] } constexpr auto begin() const requires input_range<const V> && input_range<range_reference_t<const V>> && is_reference_v<range_reference_t<const V>> { return iterator<true>{*this, ranges::begin(base_)}; } constexpr auto end() { […] } constexpr auto end() const requires input_range<const V> && input_range<range_reference_t<const V>> && is_reference_v<range_reference_t<const V>> { if constexpr (forward_range<const V> && forward_range<range_reference_t<const V>> && common_range<const V> && common_range<range_reference_t<const V>>) return iterator<true>{*this, ranges::end(base_)}; else return sentinel<true>{*this}; } }; […] }Modify 26.7.15.2 [range.join.with.view], class template join_with_view synopsis, as indicated:
namespace std::ranges { […] template<input_range V, forward_range Pattern> requires view<V> && input_range<range_reference_t<V>> && view<Pattern> && compatible-joinable-ranges<range_reference_t<V>, Pattern> class join_with_view : public view_interface<join_with_view<V, Pattern>> { private: […] public: […] constexpr auto begin() { […] } constexpr auto begin() const requires input_range<const V> && forward_range<const Pattern> && input_range<range_reference_t<const V>> && is_reference_v<range_reference_t<const V>> { return iterator<true>{*this, ranges::begin(base_)}; } constexpr auto end() { […] } constexpr auto end() const requires input_range<const V> && forward_range<const Pattern> && input_range<range_reference_t<const V>> && is_reference_v<range_reference_t<const V>> { using InnerConstRng = range_reference_t<const V>; if constexpr (forward_range<const V> && forward_range<InnerConstRng> && common_range<const V> && common_range<InnerConstRng>) return iterator<true>{*this, ranges::end(base_)}; else return sentinel<true>{*this}; } }; […] }
[2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: New → Resolved.]
Proposed resolution: