Proposed resolution:

This wording is relative to N4861.

1. Modify [memory.syn] as indicated:

   […]
   template<class T, class D>
     bool operator>=(nullptr_t, const unique_ptr<T, D>& y);
   template<class T, class D>
     requires three_way_comparable_with<typename unique_ptr<T, D>::pointer, nullptr_t>
     compare_three_way_result_t<typename unique_ptr<T, D>::pointer, nullptr_t>
       operator<=>(const unique_ptr<T, D>& x, nullptr_t);
   […]
   

2. Modify [unique.ptr.special] as indicated:

   template<class T, class D>
     requires three_way_comparable_with<typename unique_ptr<T, D>::pointer, nullptr_t>
     compare_three_way_result_t<typename unique_ptr<T, D>::pointer, nullptr_t>
       operator<=>(const unique_ptr<T, D>& x, nullptr_t);
   

   -18- Returns: compare_three_way()(x.get(), static_cast<typename unique_ptr<T, D>::pointer>(nullptr)).

[ 2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP. ]

[ 2020-04-18 Issue Prioritization ]

Status set to Tentatively Ready after seven positive votes on the reflector.

[ 2020-04-14; Replacing the functional cast by a static_cast as result of reflector discussion ]

The constraint for operator<=>(const unique_ptr<T, D>&, nullptr_t) cannot be satisfied, because std::three_way_comparable<nullptr_t> is false, because nullptr <=> nullptr is ill-formed.

We can make it work as intended by comparing x.get() to pointer(nullptr) instead of to nullptr directly.