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3873.join_with_view's const begin is underconstrained
Section: 25.7.15.2 [range.join.with.view] Status: New Submitter: Hewill Kang Opened: 2023-02-04 Last modified: 2023-02-10
Priority: 3
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Discussion:
In order to ensure that the pattern range is compatible with the inner range, join_with_view requires that the two range types must satisfy compatible-joinable-ranges, which requires that the value type, reference type, and rvalue reference type of the two range types share a common type.
const-qualified, there is no guarantee that their common reference type still exists, in which case a hard error may occur since join_with_view's const begin does not check for this (online example):
#include <ranges>
struct S {
S(const int&);
S(int&&);
S(const int&&) = delete;
};
int main() {
const auto r = std::views::single(std::views::single(0))
| std::views::join_with(std::views::single(S{0}));
auto e = std::ranges::iter_move(r.begin()); // hard error
}
[Issaquah 2023-02-10; LWG issue processing]
Set priority to 3.
Proposed resolution:
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