Hi all,

I would like to propose a very simple addition to the standard library in order to simplify variadic parameter pack expansion, and before writing a formal proposal I just wanted to share it in order to get feedback.

Typically, expanding indices sequences require writing a dedicated helper wrapper. This is tedious and offers no value at all. A Search through the Internet shows that similar approaches have been informally proposed, for instance, in [stack overflow](https://stackoverflow.com/questions/47210956/c17-multiple-parameter-pack-expansion)

The proposal is about having a generic, reusable wrapper for expanding integer sequences similar to the std::apply facility for expanding tuples, considering the following:
 - simplify the common case of expanding an index sequence with a provided lambda
 - expansion must be usable as literal values to enable template instantiation (for instance, can be used to call std::get<index>(x)...)
 - support for non-sequential index sequences, and integral types other than std::size_t
 - syntax similar to std::apply and std::visit
 - the name could be just std::apply as a new overload, as this is conceptually the same as expanding a tuple but for integral constants

The proposal is then:

/// Invoke the Callable object function with a sequence of indices
/// \param[in]  function    A callable object that will be invoked with std::integral_constant arguments instantiated for each index in the \a sequence integer_sequence
/// \param[in]  sequence    A std::integer_sequence to be expanded
/// \return     The \a apply helper function returns whatever the provided callable object returns
namespace std
{
  template<typename F, typename T, T... indices>
  constexpr decltype(auto) apply(F&& function, [[maybe_unused]] std::integer_sequence<T, indices...> sequence)
  {
    return function(std::integral_constant<T, indices>{}...);
  }
}

The idea of having arguments encoded as std::integral_constant allows for using them not only as constexpr but also as template arguments.

This simple helper function could allow us to write, for instance:

/// performs the addition of two arrays of arithmetic types
template<typename T, std::size_t N>
auto operator +(const std::array<T, N>& lhs, const std::array<T, N>& rhs)
  -> std::enable_if_t<std::is_arithmetic_v<T>, std::array<T, N>>
{
  return std::apply([&](auto... indices) { return std::array<T, N>{ (std::get<indices>(lhs) + std::get<indices>(rhs))... }; }, std::make_index_sequence<N>{});
}

int main(int argc, char** argv)
{
    auto result = std::array{1, 2, 3} + std::array{5, 4, 3};
    apply([&](auto... indices) { (std::cout << ... << result[indices]); }, std::make_index_sequence<3>{});
}

Even the current std::apply function for tuples could be rewriten on top of this proposal.

No language changes are required. It can be implemented on top of C++17 and C++14.

Waiting for your feedback,

Jordi