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`transform_iterator` does not work with `std::sort` from libc++ 22

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
58/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
cpp
Domain
tooling

Research direction

Start with boost/iterator/transform_iterator.hpp and the repro.cpp example, then compare the transform_iterator behavior used by std::sort with the libc++ 22 diagnostic and the linked LLVM issue. Done means the provided C++17 example compiles successfully with libc++ 22 and sorts the wrapped values.

Written by the indexing model from the issue text.

Description

If the comparator has a templated operator(), sorting through a boost::transform_iterator fails to compile with libc++ 22.

#include <algorithm>
#include <string>
#include <vector>
#include <boost/iterator/transform_iterator.hpp>

template <typename T> struct Wrapper { T item; };

struct Identity {
  Wrapper<std::string> &operator()(Wrapper<std::string> &w) const { return w; }
};

struct Cmp {
  template <typename T>
  bool operator()(const Wrapper<T> &a, const Wrapper<T> &b) const { return a.item < b.item; }
};

int main() {
  std::vector<Wrapper<std::string>> v{{"b"}, {"a"}};
  typedef boost::transform_iterator<Identity, std::vector<Wrapper<std::string>>::iterator> It;
  std::sort(It(v.begin()), It(v.end()), Cmp());
}

$ clang++ -std=c++17 -stdlib=libc++ -c repro.cpp
...
/usr/bin/../include/c++/v1/__algorithm/sift_down.h:55:7: error: no matching function for call to object of type
      'Cmp'
   55 |   if (__comp(__first[__child], __first[__start]))
      |       ^~~~~~
repro.cpp:14:8: note: candidate template ignored: could not match 'Wrapper' against 'operator_brackets_proxy'
   14 |   bool operator()(const Wrapper<T> &a, const Wrapper<T> &b) const { return a.item < b.item; }
      |        ^
...

See also https://github.com/llvm/llvm-project/issues/223290.

Dominant language
C++
Stars
23
Forks
70
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