Compile error when using flat_map::extract_sequence with small_vector

[arthaud]

Hi,

My project started seeing compilation errors with boost 1.86.0, when using flat_map::extract_sequence in combination with small_vector.

Here is a repro:

#include <boost/container/flat_map.hpp>
#include <boost/container/small_vector.hpp>
#include <iostream>

using SmallVector = boost::container::small_vector<std::pair<int, int>, 1>;
using FlatMap = boost::container::flat_map<int, int, std::less<int>, SmallVector>;

int main()
{
    FlatMap map;
    map.insert_or_assign(1, 1);

    auto container = map.extract_sequence();
    map.adopt_sequence(boost::container::ordered_unique_range,
        std::move(container));

    std::cout << "size: " << map.size() << "\n";
    return 0;
}

This results in:

/Users/arthaud/homebrew/opt/boost/include/boost/container/flat_map.hpp:77:11: error: non-const lvalue reference to type 'small_vector<[2 * ...], void, [...]>' cannot bind to a value of unrelated type 'small_vector<[2 * ...], boost::container::new_allocator<std::pair<int, int>>, [...]>'
{  return s; }
          ^
/Users/arthaud/homebrew/opt/boost/include/boost/container/flat_map.hpp:1605:31: note: in instantiation of function template specialization 'boost::container::dtl::force<boost::container::small_vector<std::pair<int, int>, 1>, boost::container::small_vector<std::pair<int, int>, 1, boost::container::new_allocator<std::pair<int, int>>>>' requested here
      return boost::move(dtl::force<sequence_type>(m_flat_tree.get_sequence_ref()));
                              ^
test.cpp:13:26: note: in instantiation of member function 'boost::container::flat_map<int, int, std::less<int>, boost::container::small_vector<std::pair<int, int>, 1>>::extract_sequence' requested here
    auto container = map.extract_sequence();
                         ^
In file included from test.cpp:1:
/Users/arthaud/homebrew/opt/boost/include/boost/container/flat_map.hpp:77:11: error: non-const lvalue reference to type 'small_vector<[2 * ...], boost::container::new_allocator<std::pair<int, int>>, [...]>' cannot bind to a value of unrelated type 'small_vector<[2 * ...], void, [...]>'
{  return s; }
          ^
/Users/arthaud/homebrew/opt/boost/include/boost/container/flat_map.hpp:1627:90: note: in instantiation of function template specialization 'boost::container::dtl::force<boost::container::small_vector<std::pair<int, int>, 1, boost::container::new_allocator<std::pair<int, int>>>, boost::container::small_vector<std::pair<int, int>, 1>>' requested here
   {  this->m_flat_tree.adopt_sequence_unique(ordered_unique_range_t(), boost::move(dtl::force<impl_sequence_type>(seq)));  }
                                                                                         ^
test.cpp:14:9: note: in instantiation of member function 'boost::container::flat_map<int, int, std::less<int>, boost::container::small_vector<std::pair<int, int>, 1>>::adopt_sequence' requested here
    map.adopt_sequence(boost::container::ordered_unique_range,
        ^
2 errors generated.

This is also visible in CI here: https://github.com/facebook/mariana-trench/actions/runs/10595093691/job/29360092181

I think this is related to dtl::force<impl_sequence_type>, but I didn't dig deeper.
FYI this issue is not present with older version (for instance, boost 1.77.0).

[igaztanaga]

Many thanks for the issue. Trying to reproduce it I found that extract/adopt were not properly tested and in general, flat_map tests needed to test more combinations of types (move-only, etc) and underlying containers (small_vector, devector, etc.). I've committed those changes first (daa81ed) and then commit changes to fix your bug.

So your bug report has been very useful, thanks!