Proposed resolution:

This wording is relative to N4981.

1. Modify [refwrap.general] as indicated:

   
      // [refwrap.comparisons], comparisons
      friend constexpr bool operator==(reference_wrapper, reference_wrapper);
      friend constexpr bool operator==(reference_wrapper, const T&);
      friend constexpr bool operator==(reference_wrapper, reference_wrapper<const T>);
   
      friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper, reference_wrapper);
      friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper, const T&);
      friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper, reference_wrapper<const T>);
   

2. Modify [refwrap.comparisons] as indicated:

   
   friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper y);
   

   -?- Constraints: The expression synth-three-way(x.get(), y.get()) is well-formed.

   -7- Returns: synth-three-way(x.get(), y.get()).

   
   friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, const T& y);
   

   -?- Constraints: The expression synth-three-way(x.get(), y) is well-formed.

   -8- Returns: synth-three-way(x.get(), y).

   
   friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper<const T> y);
   

   -9- Constraints: is_const_v<T> is `false`. The expression synth-three-way(x.get(), y.get()) is well-formed.

   -10- Returns: synth-three-way(x.get(), y.get()).

[ St. Louis 2024-06-29; Status changed: Voting → WP. ]

[ 2024-05-08; Reflector poll ]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

p2944r3 added these hidden friends to `reference_wrapper`:

   friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, reference_wrapper);
   friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, const T&);
   friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, reference_wrapper<const T>);

These functions are not templates, and so their declarations are ill-formed for any type that does have any comparison operators, e.g.

    struct A { } a;
    std::reference_wrapper<A> r(a);

Instantiating reference_wrapper<A> will instantiate the declarations of the hidden friends, which will attempt to determine the return types of the operator<=> functions. That fails because `synth-three-way` is constrained and can't be called with arguments of type `A`.

This can be solved by changing those functions into templates, so they aren't instantiated eagerly, e.g.,

    template<class U = T>
    friend constexpr synth-three-way-result<TU> operator<=>(reference_wrapper, reference_wrapper);

    friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper y)
    requires requires (const T t) { synth-three-way(t, t); }

In practice the requires-clause can be implemented more simply (and efficiently) by checking the constraints of synth-three-way directly:

    requires (const T t) { { t < t } -> boolean-testable; }

The proposed resolution has been committed to libstdc++'s master branch.