TypeList.cpp

#include <type_traits>
#include <iostream>

struct Foo {};

template <typename... Args>
struct TypeList {};

template <int N, typename Target>
constexpr int searchTypeImpl() {
    return -1;
}

template <int N, typename Target, typename Next, typename... Rest>
constexpr int searchTypeImpl() {
    if constexpr (std::is_same_v<Target, Next>) {
        return N;
    }

    return searchTypeImpl<N + 1, Target, Rest...>();
}

template <typename Target, template<typename...> typename TList, typename... Args>
constexpr int findType([[maybe_unused]] TList<Args...> arg) {
    return searchTypeImpl<0, Target, Args...>();
}

int main() {
    TypeList<bool, char, int, long> integrals;

    static_assert(findType<int>(integrals) == 2, "Unexpected index of int type!");
    static_assert(findType<Foo>(integrals) == -1, "Unexpected index of non-included type!");
}

TODO: add a check for empty type sequences in findType().