[Bug libstdc++/80662] libstdc++ basic_string casting oddity

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80662

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Further reduced:

namespace std
{
  // <string>
  struct string
  {
    string(const char* s) : str(s) { }

    const char* str;
  };

  // <ostream>
  template<typename C>
    struct basic_ostream
    {
      basic_ostream& operator<<(const char*)
      {
        return *this;
      }
    };

  using ostream = basic_ostream<char>;

  inline ostream
  operator<<(ostream& os, const string& s)
  {
    os << s.str;
    return os;
  }

#ifndef UNCONSTRAINED
  // GCC 7 code

  template<typename _Tp>
    struct __is_convertible_to_basic_ostream
    {
      template<typename _Ch>
      static basic_ostream<_Ch>& __check(basic_ostream<_Ch>*);

      static void __check(void*) = delete;

      using ostream_type = decltype(__check((_Tp*)0));
    };

  template<typename _Tp>
    struct __is_convertible_to_basic_ostream<_Tp&>
    {
    };

  template<typename _Ostream, typename _Tp>
    inline typename __is_convertible_to_basic_ostream<_Ostream>::ostream_type
    operator<<(_Ostream&& __os, const _Tp& __x)
    {
      __os << __x;
      return __os;
    }
#else
  // GCC 6 code

  template<typename _CharT, typename _Traits, typename _Tp>
    inline basic_ostream<_CharT, _Traits>&
    operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
    {
      __os << __x;
      return __os;
    }
#endif
}

struct my_stream : public std::ostream {
    template<typename T>
    my_stream& operator<<(T&& value)
    {
        std::ostream::operator<<(value);
        return *this;
    }
};

int main()
{
    my_stream& s = (my_stream{} << "hello world");
}