[compare.type]

There is an implementation-defined total ordering of all types.

For any (possibly incomplete) types X and Y, the expression TYPE-ORDER(X, Y) is a constant expression ([expr.const.const]) of type strong_ordering ([cmp.strongord]).

Its value is strong_ordering::less if X precedes Y in this implementation-defined total order, strong_ordering::greater if Y precedes X, and strong_ordering::equal if they are the same type.

[Note 1:

int, const int and int& are different types.

— end note]

[Note 2:

This ordering need not be consistent with the one induced by type_info::before.

— end note]

[Note 3:

The ordering of TU-local types from different translation units is not observable, because the necessary specialization of type_order is impossible to name.

— end note]

🔗

template<class T, class U> struct type_order {
  static constexpr strong_ordering value = TYPE-ORDER(T, U);

  using value_type = strong_ordering;

  constexpr operator value_type() const noexcept { return value; }
  constexpr value_type operator()() const noexcept { return value; }
};

17 Language support library [support]

17.12 Comparisons [cmp]

17.12.7 Type Ordering [compare.type]