Check if a type is passed in variadic template parameter pack

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You’re looking for std::disjunction. It’s specified in N4564 [meta.logical].

#include <type_traits>

template<typename T, typename... Ts>
constexpr bool contains()
{ return std::disjunction_v<std::is_same<T, Ts>...>; }

static_assert(    contains<int,      bool, char, int, long>());
static_assert(    contains<bool,     bool, char, int, long>());
static_assert(    contains<long,     bool, char, int, long>());
static_assert(not contains<unsigned, bool, char, int, long>());

Live demo

Or, adapted to a struct

template<typename T, typename... Ts>
struct contains : std::disjunction<std::is_same<T, Ts>...>
{};

Or, using fold expressions

template<typename T, typename... Ts>
struct contains : std::bool_constant<(std::is_same<T, Ts>{} || ...)>
{};

Live demo

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