std::relation - cppreference.com (original) (raw)
template< class R, class T, class U > concept relation = std::predicate<R, T, T> && std::predicate<R, U, U> && std::predicate<R, T, U> && std::predicate<R, U, T>; (1) (since C++20)

The concept relation<R, T, U> specifies that R defines a binary relation over the set of expressions whose type and value category are those encoded by either T or U.

[edit] References