-Wconversion warning with is_signed for enums

[pdimov]

..\../boost/type_traits/is_signed.hpp: In instantiation of 'const no_cv_t boost::detail::is_signed_values<test::enum1>::minus_one':
..\../boost/type_traits/is_signed.hpp:45:4:   required from 'const bool boost::detail::is_signed_helper<test::enum1>::value'
..\../boost/type_traits/is_signed.hpp:74:4:   required from 'const bool boost::detail::is_signed_impl<test::enum1>::value'
..\../boost/type_traits/is_signed.hpp:79:27:   required from 'struct boost::is_signed<test::enum1>'
..\../boost/container_hash/hash.hpp:178:135:   required from 'typename boost::enable_if_<boost::is_enum<T>::value, long unsigned int>::type boost::hash_value(T) [with T = test::enum1; typename boost::enable_if_<boost::is_enum<T>::value, long unsigned int>::type = long unsigned int]'
..\../boost/container_hash/hash.hpp:672:30:   required from 'std::size_t boost::hash<T>::operator()(const T&) const [with T = test::enum1; std::size_t = long unsigned int]'
test\hash_enum_test.cpp:39:5:   required from here
..\../boost/type_traits/is_signed.hpp:37:59: error: the result of the conversion is unspecified because '-1' is outside the range of type 'boost::detail::is_signed_values<test::enum1>::no_cv_t' {aka 'test::enum1'} [-Werror=conversion]
   37 |    static const no_cv_t minus_one = (static_cast<no_cv_t>(-1));
      |                                                           ^~

The warning is correct, because -1 is not necessarily a valid value for enumeration types, so this is not required to work.

I'm not sure how we can fix this. I tried a few things, but nothing worked.

is_signed is not particularly useful for enums, and in fact std::is_signed doesn't support them, so this may seem of low priority; but what I'm actually trying to implement is underlying_type, and its C++03 implementation is

conditional<is_signed<T>, make_signed<T>, make_unsigned<T>>::type::type

Maybe what we actually need is just boost::underlying_type. (Or boost::underlying_type_, because Core has already - and incorrectly - claimed boost::underlying_type in boost/core/scoped_enum.hpp.)

[Lastique]

boost::underlying_type in boost/core/underlying_type.hpp does support scoped enums from boost/core/scoped_enum.hpp but it also supports native enums if std::underlying_type is available. So, if you're going to implement a generic C++03 version of underlying_type it should go under the name of boost::underlying_type. No need to add another type trait that does the same thing.