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std::relation

Defined in header <concepts> 
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.

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