g++: template member specialization oddity

[Jason Riedy <ejr at cise dot ufl dot edu>]

I was experimenting with template member specialization when I
ran into a bit of an oddity.  The following code compiles and
does what I expect when the two instances of ack<> have the
same type, but not when they have different types.

I'm not positive my code's right (think it is), but I'm certain 
it either shouldn't compile at all or should compile even when 
the ack<> template is instantiated with two different types.

Jason

#include <iostream>

template <class T>
class ack
{
public:
  T v;

  template <class S>
  S get_v(S s) const { return v + s; }
};

template<class T>
template<>
int
ack<T>::get_v(int s) const
{
  return v + 1;  // ignore s if S == int
}

int
main (void)
{
  ack<int> a;
  ack<long> b; // change long to int and it'll compile & work...

  a.v = 4;
  b.v = 4;

  cout << "a " << a.get_v(4)
       << " b " << b.get_v(4) << endl;
}