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4122. Ill-formed operator<=> can cause hard error when instantiating std::inplace_vector

Section: 23.3.14.1 [inplace.vector.overview] Status: New Submitter: Jiang An Opened: 2024-07-20 Last modified: 2024-08-02

Priority: 2

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Discussion:

This is almost the same problem as LWG 4071(i) 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>.

[2024-08-02; Reflector poll]

Set priority to 2 after reflector poll.

Proposed resolution:

This wording is relative to N4986.

  1. Modify 23.3.14.1 [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); }
  };
};