std::relation
Min standard notice:
Header: <concepts>
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.
# Declarations
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>;
(since C++20)