Proposed resolution:

Add a paragraph to [futures.unique_future]:

R&& unique_future::get();
R& unique_future<R&>::get();
void unique_future<void>::get();

Note:...

Effects: If is_ready() would return false, block on the asynchronous result associated with *this.

Synchronization: if *this is associated with a promise object, the completion of set_value() or set_exception() to that promise happens before (1.10) get() returns.