I'm getting E:\C++Libs\boost_1_59_0\boost/multi_index/composite_key.hpp(638): error C2535: 'boost::multi_index::composite_key_result<boost::multi_index::composite_key<const vst::Foo1::Bar2::Baz3,boost::multi_index::const_mem_fun<vst::Foo1::Bar2::Baz3,UID,UID vst::Foo1::Bar2::Baz3::owner(void) const>,boost::multi_index::member<vst::Foo1::Bar2::Baz3,const uint32_t,pointer-to-member(0x18)>,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type>> boost::multi_index::composite_key<const vst::Foo1::Bar2::Baz3,boost::multi_index::const_mem_fun<vst::Foo1::Bar2::Baz3,UID,UID vst::Foo1::Bar2::Baz3::owner(void) const>,boost::multi_index::member<vst::Foo1::Bar2::Baz3,const uint32_t,pointer-to-member(0x18)>,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type>::operator ()(const boost::reference_wrapper<const Class> &) const': member function already defined or declared with [ Class=vst::Foo1::Bar2::Baz3 ] (compiling source file Bar2\Delta.cpp) (actual names from my code have been sanitized) Using Boost 1.59 and VS2015. The function it points to is result_type operator()(const reference_wrapper<value_type>& x)const but the error message looks like it is duplicating the previous function in the file, result_type operator()(const reference_wrapper<const value_type>& x)const and the chain of instantiation comes through composite_key.hpp, detail/ord_index_args.hpp(46), mpl/eval_if.hpp(41), detail/ord_index_args.hpp(70), ordered_index.hpp(65) with [ Arg1=boost::multi_index::tag<UID,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na,boost::mpl::na>, Arg2=boost::multi_index::composite_key<const vst::Foo1::Bar2::Baz3,boost::multi_index::const_mem_fun<vst::Foo1::Bar2::Baz3,UID,UID vst::Foo1::Bar2::Baz3::owner(void) const>,boost::multi_index::member<vst::Foo1::Bar2::Baz3,const uint32_t,pointer-to-member(0x18)>,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type,boost::tuples::null_type>, Arg3=boost::mpl::na ] What's going on here? This worked previously with Boost 1.55 and VS2010.