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3820.cartesian_product_view::iterator::prev is not quite right
Section: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-11-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.cartesian.iterator].
View all issues with C++23 status.
Discussion:
Currently, cartesian_product_view::iterator::prev has the following Effects:
auto& it = std::get<N>(current_);
if (it == ranges::begin(std::get<N>(parent_->bases_))) {
it = cartesian-common-arg-end(std::get<N>(parent_->bases_));
if constexpr (N > 0) {
prev<N - 1>();
}
}
--it;
which decrements the underlying iterator one by one using recursion. However, when N == 0, it still detects if the first iterator has reached the beginning and assigns it to the end, which is not only unnecessary, but also causes cartesian-common-arg-end to be applied to the first range, making it ill-formed in some cases, for example:
#include <ranges>
int main() {
auto r = std::views::cartesian_product(std::views::iota(0));
r.begin() += 3; // hard error
}
This is because, for the first range, cartesian_product_view::iterator::operator+= only requires it to model random_access_range. However, when x is negative, this function will call prev and indirectly calls cartesian-common-arg-end, since the latter constrains its argument to satisfy cartesian-product-common-arg, that is, common_range<R> || (sized_range<R> && random_access_range<R>), which is not the case for the unbounded iota_view, resulting in a hard error in prev's function body.
if constexpr so that we just decrement the first iterator and nothing else.
[Kona 2022-11-08; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify [ranges.cartesian.iterator] as indicated:
template<size_t N = sizeof...(Vs)> constexpr void prev();-6- Effects: Equivalent to:
auto& it = std::get<N>(current_); if constexpr (N > 0) { if (it == ranges::begin(std::get<N>(parent_->bases_))) { it = cartesian-common-arg-end(std::get<N>(parent_->bases_)); if constexpr (N > 0) { prev<N - 1>(); } } } --it;
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