How to use enum class values as part of for-loop?

I would recommend doing something different. Create a vector of Suit and one to Rank, and loop over them using the power of STL

const std::vector<Suit> v_suit {Suit::clubs, Suit::diamonds, Suit::hearts, Suit::spades};

const std::vector<Rank> v_rank {Rank::one, Rank::two, Rank::three, Rank::four, Rank::five, 
                          Rank::six, Rank::seven, Rank::eight, Rank::nine, Rank::ten, Rank::jack, 
                          Rank::queen, Rank::king, Rank::ace};

Yes, you have to type them twice, but this permits you to use whatever values you want for them (ie. not consecutive), not use awkward stuff like enum_count (What card do you want? Give me a diamonds enum_count!!), no need for casting, and use the iterators provided to std::vector.

To use them:

for(const auto & s : v_suit)
    for (const auto & r : v_rank)
        cards.push_back({s,r});

You could cast your suit and rank variables to an int& and increase them as such.

    for (Suit suit = Suit::clubs; suit < Suit::enum_count; ((int&)suit)++) {
        for (Rank rank = Rank::one; rank < Rank::enum_count; ((int&)rank)++) {

Yet this might cause some problems like when you assign values to your enum entries.

---

You could also create a little function that does this for you with the correct type:

template <typename T>
T& increment(T& value)
{
    static_assert(std::is_integral<std::underlying_type_t<T>>::value, "Can't increment value");
    ((std::underlying_type_t<T>&)value)++;
    return value;
}

Deck::Deck() {
    bool a = std::is_integral<std::underlying_type_t<Suit>>::value;

    // ERROR ON THE BELOW LINE
    for (Suit suit = Suit::clubs; suit < Suit::enum_count; increment(suit)) {
        for (Rank rank = Rank::one; rank < Rank::enum_count; increment(rank)) {
            Card created_card;
            created_card.suit = suit;
            created_card.rank = rank;
            cards.push_back(created_card);
        };
    };
};