Cannot use boost::multi_index::key with `const& noexcept` member function

[phimuemue]

The following program does not compile (https://wandbox.org/permlink/odsAUNRroBveTxqw). (I tested clang 8.0.0 and g++ 9.1.0.)

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/key.hpp>

struct S final {
    int m_n;
    S(int n) noexcept : m_n(n) {}
    int Get() const & noexcept { // removing "& noexcept" makes it compile
        return m_n;
    }
};

int main() {
    boost::multi_index::multi_index_container<
        S,
        boost::multi_index::indexed_by<
            boost::multi_index::ordered_unique<
                boost::multi_index::key<&S::Get>
            >
        >
    > container;
}

clang 8.0.0 reports:

In file included from prog.cc:3:
/opt/wandbox/boost-1.70.0/clang-8.0.0/include/boost/multi_index/key.hpp:77:22: error: implicit instantiation of undefined template 'boost::multi_index::detail::typed_key_impl<int (S::*)() const & noexcept, &S::Get, void>'
struct key_impl<Key>:typed_key_impl<decltype(Key),Key>{};
                     ^
/opt/wandbox/boost-1.70.0/clang-8.0.0/include/boost/multi_index/key.hpp:134:23: note: in instantiation of template class 'boost::multi_index::detail::key_impl<&S::Get>' requested here
  using type=typename key_impl<Keys...>::type;
                      ^
/opt/wandbox/boost-1.70.0/clang-8.0.0/include/boost/multi_index/key.hpp:140:1: note: in instantiation of template class 'boost::multi_index::detail::limited_size_key_impl<&S::Get>' requested here
using key=typename detail::limited_size_key_impl<Keys...>::type;
^
prog.cc:18:37: note: in instantiation of template type alias 'key' requested here
                boost::multi_index::key<&S::Get>
                                    ^
/opt/wandbox/boost-1.70.0/clang-8.0.0/include/boost/multi_index/key.hpp:38:8: note: template is declared here
struct typed_key_impl;
       ^
prog.cc:20:9: error: expected a type
        >
        ^
prog.cc:21:5: error: expected a type
    > container;
    ^
prog.cc:21:7: error: C++ requires a type specifier for all declarations
    > container;
      ^
4 errors generated.

I suspect that the problem is that key_impl is not specialized for const& and/or noexcept member functions.

[joaquintides]

Thank you for your report. I've added support for volatile/ref-qualified/noexcept memfuns in e49ae2b. Could you please test locally if your problem is solved?

[phimuemue]

Thank you. I compiled it locally with Visual Studio, and could not observe any problems, albeit the compiler warns:

key.hpp(90): warning C4003: not enough arguments for function-like macro invocation 'BOOST_MULTI_INDEX_KEY_REMOVE_MEMFUN_NOEXCEPT'`

[joaquintides]

Warning fixed in a7ef250.

Best regards,