When to use forwarding reference in ranged for (C++17)

It has to do with lvalues and rvalues. This is a really good, easy to understand introduction to them in my opinion - Understanding lvalues and rvalues in C and C++[^]. Another good one is this - Rvalue references[^]. It's a little denser but it goes into more details (e.g. reference collapsing, move semantics).

Lastly, I'm not 100% but I wanna say when using auto with a reference or pointer it retains the cv-qualifier.

I started investigating the answer to your question thinking I understand references. It turned out they are more complicated than I thought.

I wanted to say that, looking at your code, it depends what you want to do with the item_r and item_rr. Being a rvalue reference, I would have expected that you cannot say something like:

template <class T>
T do_something_rval (std::vector<T>& container) {
  T t = 0;
  for (auto&& item_rr : container) {
    // do something with item
    t += item_rr;
    item_rr = t; // Expected an error here
  }
  return t;
}

However I didn't get any error neither on definition nor on instantiation (my compiler is MSVC++ 19 updated with latest patches):

int main ()
{
  vector<int> v1 = { 1, 2, 3, 4, 5 };
  int sum_rval = do_something_rval (v1);
  cout << "v1";
  for (auto i : v1)
    cout << " " << i;
  cout << " sum=" << sum_rval << endl;

And the output:

v1 1 3 6 10 15 sum=15

shows that you can modify an object through a rvalue reference.

If, however, I change the 'auto' to an explicit type:

template <class T>
T do_something_rval (std::vector<T>& container) {
  T t = 0;
  for (int&& item_rr : container) {
    // do something with item
    t += item_rr;
    item_rr = t;
  }
  return t;
}

I get an error:

Error C2440 'initializing': cannot convert from 'int' to 'int &&'

It goes to show that for auto declarations the compiler drops any rvalue reference specifications.

My conclusion: there is no point in using rvalue reference for auto declarations.