The following code fails to compile.

    #include <utility>

    template <typename T, typename Seq>
    struct X;

    template <typename T>
    struct X<T, std::make_index_sequence<sizeof(T)>> {
    };

    X<char, std::make_index_sequence<1>> x;
    // ^^^
    // error: implicit instantiation of undefined template
    // 'X<char, std::integer_sequence<unsigned long, 0>>

See https://godbolt.org/z/o8ejrTGKE

Replacing make_index_sequence<1> with the equivalent index_sequence<0> in the class template definition works around the issue.