How to make a variadic is_same?

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Nice and concise with C++17:

template <class T, class... Ts>
struct is_any : std::disjunction<std::is_same<T, Ts>...> {};

And the dual:

template <class T, class... Ts>
struct are_same : std::conjunction<std::is_same<T, Ts>...> {};

A variation that uses fold expressions:

template <class T, class... Ts>
struct is_any : std::bool_constant<(std::is_same_v<T, Ts> || ...)> {};

template <class T, class... Ts>
struct are_same : std::bool_constant<(std::is_same_v<T, Ts> && ...)> {};

Or as C++20 concepts:

template <typename T, typename... Ts>
concept is_any = std::disjunction_v<std::is_same<T, Ts>...>;

template <typename T, typename... Ts>
concept are_same = std::conjunction_v<std::is_same<T, Ts>...>;

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