Error when casting temporary object to non-const reference

Yes, I think that is a bug in libc++.

According to [ostream.rvalue] there is an overload:

template<class charT, class traits, class T>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);

But libc++ implements this similar to:

template <class _Stream, class _Tp>
enable_if_t</*...*/, _Stream&&>
operator<<(_Stream&& __os, const _Tp& __x)
{
    // ...
}

The implementation makes this overload a better candidate than the in-class operator<< of ostream in overload resolution if a stream rvalue is used with <<, while the signature in the standard wouldn't be, and it also returns a rvalue reference, when it should return a lvalue reference. It also returns a reference of same type as was passed to it, while it should return a reference to its ostream base class, according to the standard quote.

The rvalue reference can not be bound to a non-const lvalue reference, hence the error.

The bug has already been reported here and there is an open LWG issue regarding the behavior here, which seems to suggest that in the future the standard might be adjusted to mandate libc++'s current behavior.