fixing build error: undefined symbol: void boost::hash_combine

[hanw]

I'm unable to build p4c on Ubuntu 20.04 without this change.

This is the error message.

ld.lld: error: undefined symbol: void boost::hash_combine<unsigned long long>(unsigned long&, unsigned long long const&)                                                           
>>> referenced by expression.cpp                                                                                                                                                   >>>               expression.cpp.o:(P4::IR::Constant::get(P4::IR::Type const*, boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<0u, 0u, (boost::multiprecision::cpp_integer_type)1, (boost::multiprecision::cpp_int_check_type)0, std::allocator<unsigned long long> >, (boost::multiprecision::expression_template_option)1>, P4::Util::SourceInfo)) in archive ../../ir/libir.a                                                                                                                                          >>> referenced by expression.cpp                                                                                                                                                   >>>               expression.cpp.o:(absl::lts_20240116::container_internal::raw_hash_set<absl::lts_20240116::container_internal::FlatHashMapPolicy<std::tuple<int, unsigned long, bool, boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<0u, 0u, (boost::multiprecision::cpp_integer_type)1, (boost::multiprecision::cpp_int_check_type)0, std::allocator<unsigned long long> >, (boost::multiprecision::expression_template_option)1> >, P4::IR::Constant const*>, P4::Util::Hash, std::equal_to<std::tuple<int, unsigned long, bool, boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<0u, 0u, (boost::multiprecision::cpp_integer_type)1, (boost::multiprecision::cpp_int_check_type)0, std::allocator<unsigned long long> >, (boost::multiprecision::expression_template_option)1> > >, std::allocator<std::pair<std::tuple<int, unsigned long, bool, boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<0u, 0u, (boost::multiprecision::cpp_integer_type)1, (boost::multiprecision::cpp_int_check_type)0, std::allocator<unsigned long long> >, (boost::multiprecision::expression_template_option)1> > const, P4::IR::Constant const*> > >::hash_slot_fn(void*, void*)) in archive ../../ir/libir.a     >>> referenced by expression.cpp                                                                                                                                                   >>>               expression.cpp.o:(absl::lts_20240116::container_internal::raw_hash_set<absl::lts_20240116::container_internal::FlatHashMapPolicy<std::tuple<int, unsigned long, bool, boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<0u, 0u, (boost::multiprecision::cpp_integer_type)1, (boost::multiprecision::cpp_int_check_type)0, std::allocator<unsigned long long> >, (boost::multiprecision::expression_template_option)1> >, P4::IR::Constant const*>, P4::Util::Hash, std::equal_to<std::tuple<int, unsigned long, bool, boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<0u, 0u, (boost::multiprecision::cpp_integer_type)1, (boost::multiprecision::cpp_int_check_type)0, std::allocator<unsigned long long> >, (boost::multiprecision::expression_template_option)1> > >, std::allocator<std::pair<std::tuple<int, unsigned long, bool, boost::multiprecision::number<boost::multiprecision::backends::cpp_int_backend<0u, 0u, (boost::multiprecision::cpp_integer_type)1, (boost::multiprecision::cpp_int_check_type)0, std::allocator<unsigned long long> >, (boost::multiprecision::expression_template_option)1> > const, P4::IR::Constant const*> > >::resize(unsigned long)) in archive ../../ir/libir.a 

[asl]

Neither boost::hash nor boost::hash_combine are used in p4c. Ubuntu 20.04 is on CI and it green. Likely you are having some local modifications.