Hi, I prepared a lenghty MWE which reproduces my problem with type manipulation using MP11 that for some reason looses const-qualification on a ::type result Here it is at https://godbolt.org/z/eCsakP and copied below. I hope the XXXXXX comments make sense and important static assertions in the main() at the bottom explain what is the expected result. ``` #include <boost/mp11.hpp> #include <cstdint> #include <type_traits> namespace mp11 = boost::mp11; template <typename BitField, int FirstBit, int NumBits, bool IsMutable> struct bit_range_reference { /* long definition, irrelevant */ }; template <typename BitField, typename FirstBit, typename NumBits> struct bit_range_reference_type { using type = bit_range_reference < BitField, FirstBit::value, NumBits::value, true > const; // XXXXXX Why this const gets lost with GCC 4.8 XXXXXX }; template <typename BitField, typename RangeBitSizes> class bit_range_reference_list { template <typename FirstBit, typename NumBits> using reference_type = typename bit_range_reference_type < BitField, FirstBit, NumBits >::type; // XXXXXX Should carry the const, shouldn't? GCC >4.8 do. // If RangeBitSizes is mp11::mp_list_c<int,7,7,2> // then first_bit_list will be mp11::mp_list_c<int,0,7,14,16> using first_bit_list = mp11::mp_fold_q < RangeBitSizes, mp11::mp_list<std::integral_constant<int, 0>>, mp11::mp_bind < mp11::mp_push_back, mp11::_1, mp11::mp_bind < mp11::mp_plus, mp11::mp_bind<mp11::mp_back, mp11::_1>, mp11::_2 > > >; static_assert(mp11::mp_at_c<first_bit_list, 0>::value == 0, "first bit must be 0"); public: using type = mp11::mp_transform < reference_type, mp11::mp_pop_back<first_bit_list>, RangeBitSizes >; }; using reference3_list = typename bit_range_reference_list < std::uint8_t, mp11::mp_list_c<int, 3>

::type;

int main() { using r1_t = mp11::mp_at_c<reference3_list, 0>; using r2_t = bit_range_reference<std::uint8_t, 0, 3, true>; using r2c_t= bit_range_reference<std::uint8_t, 0, 3, true> const; //#if defined(__GNUC__) && (__GNUC__ == 4) // GCC 4.8 compiles, later versions of GCC fail to compile static_assert(std::is_same<r1_t, r2_t>::value, "GCC says r1_t is const-qualified"); //#else // GCC 4.9, 5, 6, 7, 8 versions compile, GCC 4.8 fail to compile static_assert(std::is_same<r1_t, r2c_t>::value, "GCC says r1_t is NOT const-qualified"); //#endif } ``` To summary, I expect this to yield true: std::is_same < mp11::mp_at_c<reference3_list, 0> bit_range_reference<std::uint8_t, 0, 3, true> const

I got this confirmed by all GCC versions but GCC 4.8. (clang and MSVC also seem to agree with the GCC >= 4.9) Is mp11::mp_transform loosing const-qualification or have I made a mistake somewhere? I'm using Boost.MP11 from Boost 1.70 and from current develop branch Best regards, -- Mateusz Loskot, http://mateusz.loskot.net