using simd_vector = typename Vc::datapar<double, Vc::datapar_abi::fixed_size<8>>;
using int_simd_vector = typename Vc::datapar<int32_t, Vc::datapar_abi::fixed_size<8>>;

...

           const int_simd_vector theta_c_rec_squared_int =
                detail::distance_squared(cell_index_coarse, interaction_partner_index_coarse);
            const simd_vector theta_c_rec_squared =
                Vc::static_datapar_cast<double>(theta_c_rec_squared_int);

compile log:
...
/usr/include/c++/6/type_traits: In instantiation of ‘struct std::make_unsigned<double>’:
/home/pfandedd/Vc_install/include/Vc/detail/synopsis.h:269:12:   required from ‘auto Vc::v2::static_datapar_cast(const Vc::v2::datapar<T, A0>&) [with T = double; U = int; A = Vc::v2::datapar_abi::fixed_size<8>]’
/home/pfandedd/git/octotiger/src/kernels/m2m_kernel_blocked_interaction.cpp:107:72:   required from here
/usr/include/c++/6/type_traits:1831:62: error: invalid use of incomplete type ‘class std::__make_unsigned_selector<double, false, false>’
     { typedef typename __make_unsigned_selector<_Tp>::__type type; };
                                                              ^~~~
/usr/include/c++/6/type_traits:1788:11: note: declaration of ‘class std::__make_unsigned_selector<double, false, false>’
     class __make_unsigned_selector;
           ^~~~~~~~~~~~~~~~~~~~~~~~
/home/pfandedd/git/octotiger/src/kernels/m2m_kernel_blocked_interaction.cpp: In member function ‘void octotiger::fmm::m2m_kernel::blocked_interaction(const octotiger::fmm::multiindex<>&, int64_t, const octotiger::fmm::multiindex<Vc::v2::datapar<int, Vc::v2::datapar_abi::fixed_size<8> > >&, const octotiger::fmm::multiindex<>&, int64_t, const std::vector<octotiger::fmm::multiindex<> >&, size_t)’:
/home/pfandedd/git/octotiger/src/kernels/m2m_kernel_blocked_interaction.cpp:107:48: error: conversion from ‘void’ to non-scalar type ‘const simd_vector {aka const Vc::v2::datapar<double, Vc::v2::datapar_abi::fixed_size<8> >}’ requested
                 Vc::static_datapar_cast<double>(theta_c_rec_squared_int);
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
CMakeFiles/octotiger_exe.dir/build.make:974: recipe for target 'CMakeFiles/octotiger_exe.dir/src/kernels/m2m_kernel_blocked_interaction.cpp.o' failed
make[2]: *** [CMakeFiles/octotiger_exe.dir/src/kernels/m2m_kernel_blocked_interaction.cpp.o] Error 1
make[2]: Leaving directory '/home/pfandedd/git/octotiger/build_with_symbols'
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/octotiger_exe.dir/all' failed
make[1]: *** [CMakeFiles/octotiger_exe.dir/all] Error 2
make[1]: Leaving directory '/home/pfandedd/git/octotiger/build_with_symbols'
Makefile:127: recipe for target 'all' failed
make: *** [all] Error 2