C++ static template class member variable

C++ can be wierd. A non-template class member variable when declared static needs to be instantiated in a link unit e.g.

struct A{
static int a;
};

int A::a;

For a template class, it can get wierd:
template < typename T>
struct A{
static int a;
};

template < typname T>
int A< T>::a;