C++11 enum class: plus int failed to compile, why?

I've got a quick code piece:

#include<iostream>
using namespace std;
enum class A:int {
    u=1,
    v=2,
    w=3
};
template<A value>
int nextEnum(){
    return value+1;
}
int main() {
    nextEnum<A::u>();
    return 0;
}

It fails to compile:

clang++ testenum.cpp -std=c++11
testenum.cpp:10:17: error: invalid operands to binary expression ('A' and 'int')
    return value+1;

Well, I've declared A as "int" enum, why it fails to "+" with int? How to fix it?