Re: Derived class reference supplied to class reference template argument

I have trouble compiling following code
with various G++ versions (and comeau online too)

-----------------------------------------------------

struct A
{
     static int foo(void)
         {
             return 42;
         }
};

struct B : A
{
} b;

template <A& a>
struct C : A
{
     static int foo(void)
         {
             return a.foo();
         }
};

int main (void)
{
     A& a(b); // OK
     return C<b>::foo(); // Fails
}

-----------------------------------------------------

As far as I can see the standard does not
forbid passing b as a template parameter,
since
- b has external linkage
- b isa B -> b isa B& -> b isa A& (because of inheritance).

Can anybody explaing why this is illegal?