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.

On many platforms, a single CPU instruction obtains both the quotient and the
remainder, and this function may leverage that, although compilers are generally
able to merge nearby / and % where suitable.