Dynamic bitset compilation broken in boost 1.90.0?

[bredelings]

The operator- on bit iterators fails to compile in boost v1.90. Could I be missing something here?

template< typename Iterator >
BOOST_DYNAMIC_BITSET_CONSTEXPR20 std::ptrdiff_t
                                 operator-( const bit_iterator_base< Iterator > & lhs, const bit_iterator_base< Iterator > & rhs )
{
    return ( lhs.m_block_iterator - rhs.m_block_iterator ) * bit_iterator_base< Iterator >::bits_per_block
         + lhs.m_bit_index - rhs.m_bit_index;
}

The error for gcc-15 looks like:

`/path/to/installed-boost-1.90.0/include/boost/dynamic_bitset/impl/dynamic_bitset.ipp:247:18: error: ‘__gnu_cxx::__normal_iterator<const long unsigned int*, std::vector<long unsigned int> > boost::bit_iterator_base<__gnu_cxx::__normal_iterator<const long unsigned int*, std::vector<long unsigned int> > >::m_block_iterator’ is protected within this context
  247 |     return ( lhs.m_block_iterator - rhs.m_block_iterator ) * bit_iterator_base< Iterator >::bits_per_block
      |              ~~~~^~~~~~~~~~~~~~~~
...

I'm not getting errors on operator== and operator<, but it looks like they may have the same issue.

Could this be fixed for boost v1.90.1?

[sehe]

@bredelings Thanks for reporting this. I added the missing friend declaration in a PR #101

The other free-function operators weren't affected as they already had the friend declarations.