[Bug libstdc++/111948] subrange modifies a const size object

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111948

--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
This is the cause:

      constexpr
      subrange(__detail::__convertible_to_non_slicing<_It> auto __i, _Sent __s,
               __size_type __n)
      noexcept(is_nothrow_constructible_v<_It, decltype(__i)>
               && is_nothrow_constructible_v<_Sent, _Sent&>)
        requires (_Kind == subrange_kind::sized)
      : _M_begin(std::move(__i)), _M_end(__s)
      {
        if constexpr (_S_store_size)
          _M_size._M_size = __n;
      }

_M_size._M_size in the function body is already const.