//OR
template< class _T>
struct DifferentBehaviourButSimiliarInterface {
    void fn() { ... }
}

template<>
struct DifferentBehaviourButSimiliarInterface< SomeType > {
    void fn() { ... }
}

template< class _T >
struct Generic : public DifferentBehaviourButSimiliarInterface< _T > {
    void generic_call() {
        //what problems with that ?
    }
    void fn(); // behavior of this call is depend of the main _T type...
};