Recursive templates don't work as expected with static variables

Question

The code

#include <iostream>
using namespace std;

template<int n> struct Fibo { static int x; };
template<> int Fibo<0>::x = 1;
template<> int Fibo<1>::x = 1;
template<int n> int Fibo<n>::x = Fibo<n-1>::x + Fibo<n-2>::x; //marked line

int main() {
    cout << Fibo<5>::x << endl;
    cout << Fibo<4>::x << endl;
    cout << Fibo<3>::x << endl;
    cout << Fibo<2>::x << endl;
    cout << Fibo<1>::x << endl;
    cout << Fibo<0>::x << endl;

    return 0;
}

outputs

0
0
1
2
1
1

in VC++. (According to user M M. it compiles as expected in gcc). When the compiler gets to the marked line with n=5 it doesn't compile that same line again for n=4, but just treats Fibo<4>::x as if it were declared with

template<> int Fibo<4>::x; // x defaults to 0

Why is that? Why does it work as expected when using

template<int n> struct Fibo { enum { x = Fibo<n-1>::x + Fibo<n-2>::x }; };
template<> struct Fibo<0> { enum { x = 1 }; };
template<> struct Fibo<1> { enum { x = 1 }; };

instead, but not with a static variable? And how do you fix the first code (without enum)?

Answer 1

The Standard is very clear on this:

14.7.1 Implicit instantiation [temp.inst]

9 The implicit instantiation of a class template does not cause any static data members of that class to be implicitly instantiated.

All the calls in main() to your Fibo<n>::x for n > 1, are explicit instantiations, that through the Fibonnaci recursion will implicitly instantiate Fibo<n-1> and Fibo<n-2> but not their members x. This means that at those points, the static members x will be evaluated to their default initialization of 0. For n=1 and n=0, the compiler will see the explicit initialization values of 1. So effectively, you get the following computation

Fibo<5>::x --> Fibo<4>::x + Fibo<3>::x --> 0 + 0 = 0
Fibo<4>::x --> Fibo<3>::x + Fibo<2>::x --> 0 + 0 = 0
Fibo<3>::x --> Fibo<2>::x + Fibo<1>::x --> 0 + 1 = 1
Fibo<2>::x --> Fibo<1>::x + Fibo<0>::x --> 1 + 1 = 2
Fibo<1>::x --> 1
Fibo<0>::x --> 1

You need to instantiate the static member x before evaluating the Fibonacci recursion. You can do this through a static const int or enum member x, or through a function (possibly constexpr in C++11) as shown by @Jarod42.