Partial template specialization: This snipplet shows how it is possible to work with the underyling types of a std::tuple.

untuple.cc

// Author:          Julius Hülsmann
// Email:           juliusHuelsmann [at] gmail.com, julius.huelsmann [at] data-spree.com
// Creation Date:   2020-02-18
// Execution: g++-10.0 -std=c++2a untuple.cc

// This snipplet shows how it is possible to work with the underyling types of a std::tuple and
// shows a principle that can be used e.g. in oder to work with an index_sequence.

#include <tuple>

template <typename tpl> struct untuple;

template <typename T, typename... types> struct untuple<std::tuple<T, types...>> {
  using type = T; // < XXX: do some meaningful stuff
};

int main() { untuple<std::tuple<int, int, int>>::type k = 2; }