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Subject: [Boost-users] [MP11] Loosing const-qualification on type result with GCC 4.8 only
From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2019-04-25 22:57:47


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
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
            mp11::mp_list<std::integral_constant<int, 0>>,
                    mp11::mp_bind<mp11::mp_back, mp11::_1>,
    static_assert(mp11::mp_at_c<first_bit_list, 0>::value == 0,
        "first bit must be 0");
    using type = mp11::mp_transform

using reference3_list = typename bit_range_reference_list
    mp11::mp_list_c<int, 3>

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");
    // 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");

To summary, I expect this to yield true:

    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,

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