16 Language support library [language.support]

namespace std {
  class strong_equality {
    int value;  // exposition only

    // exposition-only constructor
    explicit constexpr strong_equality(eq v) noexcept : value(int(v)) {}    // exposition only

  public:
    // valid values
    static const strong_equality equal;
    static const strong_equality nonequal;
    static const strong_equality equivalent;
    static const strong_equality nonequivalent;

    // conversion
    constexpr operator weak_equality() const noexcept;

    // comparisons
    friend constexpr bool operator==(strong_equality v, unspecified) noexcept;
    friend constexpr bool operator!=(strong_equality v, unspecified) noexcept;
    friend constexpr bool operator==(unspecified, strong_equality v) noexcept;
    friend constexpr bool operator!=(unspecified, strong_equality v) noexcept;
    friend constexpr strong_equality operator<=>(strong_equality v, unspecified) noexcept;
    friend constexpr strong_equality operator<=>(unspecified, strong_equality v) noexcept;
  };

  // valid values' definitions
  inline constexpr strong_equality strong_equality::equal(eq::equal);
  inline constexpr strong_equality strong_equality::nonequal(eq::nonequal);
  inline constexpr strong_equality strong_equality::equivalent(eq::equivalent);
  inline constexpr strong_equality strong_equality::nonequivalent(eq::nonequivalent);
}