`round2neg_inf_c` invokes UB, Clang refuses to compile

[intractabilis]

numeric_conversion/include/boost/numeric/conversion/detail/converter.hpp

Line 29 in d1b479f

typedef mpl::integral_c<std::float_round_style, std::round_toward_neg_infinity> round2neg_inf_c ;

This invokes undefined behavior. Clang correctly identifies it and refuses to compile if you try to instantiate round2neg_inf_c. Here is an MWE (verified with Boost 1.89-1.90)

$ clang++ --version
clang version 21.1.6
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
$ clang++ -c -x c++ - <<EOF
#include <boost/numeric/conversion/converter.hpp>

int main (int argc, char *argv[]) {
    boost::numeric::convdetail::round2neg_inf_c x;
}
EOF
In file included from <stdin>:1:
In file included from /usr/include/boost/numeric/conversion/converter.hpp:13:
In file included from /usr/include/boost/numeric/conversion/conversion_traits.hpp:13:
In file included from /usr/include/boost/numeric/conversion/detail/conversion_traits.hpp:21:
In file included from /usr/include/boost/numeric/conversion/detail/is_subranged.hpp:17:
In file included from /usr/include/boost/mpl/multiplies.hpp:17:
In file included from /usr/include/boost/mpl/times.hpp:19:
In file included from /usr/include/boost/mpl/aux_/arithmetic_op.hpp:17:
In file included from /usr/include/boost/mpl/integral_c.hpp:32:
/usr/include/boost/mpl/aux_/integral_wrapper.hpp:62:51: error: in-class initializer for static data member is not a
      constant expression
   62 |     BOOST_STATIC_CONSTANT(AUX_WRAPPER_VALUE_TYPE, next_value = BOOST_MPL_AUX_STATIC_CAST(AUX_WRAPPER_VALUE_TYPE, (N + 1)));
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/config/detail/suffix.hpp:410:72: note: expanded from macro 'BOOST_STATIC_CONSTANT'
  410 | #     define BOOST_STATIC_CONSTANT(type, assignment) static const type assignment
      |                                                                        ^~~~~~~~~~
<stdin>:4:49: note: in instantiation of template class 'mpl_::integral_c<std::float_round_style,
      std::round_toward_neg_infinity>' requested here
    4 |     boost::numeric::convdetail::round2neg_inf_c x;
      |                                                 ^
/usr/include/boost/mpl/aux_/integral_wrapper.hpp:62:64: note: integer value 4 is outside the valid range of values
      [-4, 3] for the enumeration type 'float_round_style'
   62 |     BOOST_STATIC_CONSTANT(AUX_WRAPPER_VALUE_TYPE, next_value = BOOST_MPL_AUX_STATIC_CAST(AUX_WRAPPER_VALUE_TYPE, (N + 1)));
      |                                                                ^
/usr/include/boost/mpl/aux_/static_cast.hpp:24:47: note: expanded from macro 'BOOST_MPL_AUX_STATIC_CAST'
   24 | #   define BOOST_MPL_AUX_STATIC_CAST(T, expr) static_cast<T>(expr)
      |                                               ^
1 error generated.

It appears that Clang reserves 3 bits for std::float_round_style. This makes std::round_toward_neg_infinity + 1 to overflow.