Compile error in bulk_operate.cpp in Linux Mint 19

[iricchi]

~/FlashX/matrix/bulk_operate.cpp: In instantiation of ‘OutType fm::basic_uops_impl<InType, OutType>::uop_abs::operator()(const InType&) const [with InType = long unsigned int; OutType = long unsigned int]’:

~/FlashX/matrix/bulk_operate_impl.h:47:18: required from ‘void fm::bulk_uoperate_impl<OpType, InType, OutType>::runA(size_t, const void*, void*) const [with OpType = fm::basic_uops_impl<long unsigned int, long unsigned int>::uop_abs; InType = long unsigned int; OutType = long unsigned int; size_t = long unsigned int]’

~/FlashX/matrix/bulk_operate.cpp:87:29: error: call of overloaded ‘abs(const long unsigned int&)’ is ambiguous return (OutType) std::abs(e);