C++11 - Parameter pack question

[Sidney Marshall <sidneym@frontiernet.net>]

Hello,

The following code compiles and runs correctly but gives a warning:

----------------------------------------
#include<iostream>

using namespace std;

template<class T>
T sum(T value) {
   return value;
}

template<class T, class ... U>
auto sum(T value, U ... values) {
     return value + sum(values ...);
}

int main() {
   cout << sum(1, 2, 4.6) << endl;
   cout << sum(1, "abcde", 2) << endl;
}
--------------------------------------

$ g++ -std=c++11 pack.cpp
pack.cpp:11:31: warning: ‘sum’ function uses 
‘auto’ type specifier without trailing return type [enabled by default]
  auto sum(T value, U ... values) {

$ ./a.exe
7.6
de
-------------------------------------------

All of my attempts to use a trailing return type 
failed (and I believe should have failed according to the standard).

My two questions:

1. What possible user mistake is the warning worried about?

2. How can I write my code with better style so that there is no warning?

Thank you.

--Sidney Marshall