17 Concepts library [concepts]

17.7 Callable concepts [concepts.callable]

17.7.5 Concept Relation [concept.relation]

template<class R, class T, class U>
  concept Relation =
    Predicate<R, T, T> && Predicate<R, U, U> &&
    CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
    Predicate<R,
      common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>,
      common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>> &&
    Predicate<R, T, U> && Predicate<R, U, T>;

Let

(1.1)
r be an expression such that decltype((r)) is R,
(1.2)
t be an expression such that decltype((t)) is T,
(1.3)
u be an expression such that decltype((u)) is U, and

(1.4)

C be

  common_reference_t<const remove_reference_t<T>&,
                     const remove_reference_t<U>&>

Relation<R, T, U> is satisfied only if

(1.5)
bool(r(t, u)) == bool(r(C(t), C(u))).
(1.6)
bool(r(u, t)) == bool(r(C(u), C(t))).