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std::flat_set dispatches non-c++17 iterators to container's insert(const_iterator, InputIterator, InputIterator) #136656

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hewillk opened this issue Apr 22, 2025 · 3 comments · May be fixed by #137462
Open

std::flat_set dispatches non-c++17 iterators to container's insert(const_iterator, InputIterator, InputIterator) #136656

hewillk opened this issue Apr 22, 2025 · 3 comments · May be fixed by #137462
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libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi.

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@hewillk
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hewillk commented Apr 22, 2025 

#include <flat_set>
#include <iterator>
#include <sstream>
#include <ranges>
#include <boost/interprocess/containers/vector.hpp>
 
int main() {
  boost::container::vector<int> v;
  std::flat_set s(v);
  std::istringstream ints("0 1 1 0");
  auto r = std::ranges::subrange(std::istream_iterator<int>(ints),
                                 std::istream_iterator<int>())
         | std::views::transform([](int i) { return i * i; });
  s.insert_range(r);
}

https://godbolt.org/z/6P3rnod76
@frederick-vs-ja
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frederick-vs-ja commented Apr 22, 2025 •
edited
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See the comments in this function:

llvm-project/libcxx/include/__flat_set/utils.h

Lines 57 to 71 in 56f5bcb

template <class _Set, class _Range>
_LIBCPP_HIDE_FROM_ABI static void __append(_Set& __set, _Range&& __rng) {
if constexpr (requires { __set.__keys_.insert_range(__set.__keys_.end(), std::forward<_Range>(__rng)); }) {
// C++23 Sequence Container should have insert_range member function
// Note that not all Sequence Containers provide append_range.
__set.__keys_.insert_range(__set.__keys_.end(), std::forward<_Range>(__rng));
} else if constexpr (ranges::common_range<_Range>) {
__set.__keys_.insert(__set.__keys_.end(), ranges::begin(__rng), ranges::end(__rng));
} else {
for (auto&& __x : __rng) {
__set.__keys_.insert(__set.__keys_.end(), std::forward<decltype(__x)>(__x));
}
}
}
};

Can we just resolve this issue as invalid Resolved as invalid, i.e. not a bug because boost::container::vector<int> doesn't provide insert_range and hence doesn't meet the C++23 sequence container requirements ([sequence.reqmts])?

Or should we submit an LWG issue to make some new member functions optional for sequence containers?

CC @huixie90.

@frederick-vs-ja frederick-vs-ja added libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. and removed new issue labels Apr 22, 2025
@huixie90
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I can simply add a cpp17 iterator check on the second branch. I am be conflicted about this. On the one hand, we can simply say that boost container is not standard compliant. On the other hand, even the boost one is not standard compliant, how can users’ ones be better?

@huixie90 huixie90 linked a pull request Apr 26, 2025 that will close this issue
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@ldionne
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ldionne commented May 6, 2025

I think it would be important to file a LWG issue about this. Sure, we can fix this as an extension, but at the end of the day the Standard has to be unambiguous about what containers should be supported in flat_foo.

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[libc++] Fix flat_{multi}set insert_range huixie90/llvm-project
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@ldionne @frederick-vs-ja @llvmbot @huixie90 @hewillk