The behavior is undefined if any member function other than the destructor, the

[cppbot@botsin.space]

The behavior is undefined if any member function other than the destructor, the
move-assignment operator, or valid is called on a future that does not refer to
shared state (although implementations are encouraged to throw std::future_error
indicating no_state in this case). It is valid to move from a future object for
which valid() is false.