Proposed resolution:

This wording is relative to N4986.

1. Modify [inplace.vector.overview], class template std::inplace_vector synopsis, as indicated:

   namespace std {
     template<class T, size_t N>
     class inplace_vector {
     public:
       […]
       constexpr friend bool operator==(const inplace_vector& x,
                                        const inplace_vector& y);
       constexpr friend synth-three-way-result<T>auto
         operator<=>(const inplace_vector& x, const inplace_vector& y);
           requires requires (const T t) { synth-three-way(t, t); } 
         {
           return lexicographical_compare_three_way(x.begin(), x.end(), y.begin(), y.end(),
                                                    synth-three-way);
         }
       constexpr friend void swap(inplace_vector& x, inplace_vector& y)
         noexcept(N == 0 || (is_nothrow_swappable_v<T> &&
                             is_nothrow_move_constructible_v<T>))
       { x.swap(y); }
     };
   };
   

[ 2024-08-02; Reflector poll ]

Set priority to 2 after reflector poll.

This is almost the same problem as LWG 4071 except that it happens to std::inplace_vector. As the operator<=> overload for std::inplace_vector is a non-template function whose return type (synth-three-way-result<T>) is not deduced, when the return type is ill-formed, hard error occurs in the instantiation of the enclosing std::inplace_vector<T, N>.