C#

// Pair<T> is IComparable if and only if T is.
// new bracketing syntax is required to indicate that the constraint applies
// only to the implementation or not of the interface
class Pair<T> : (IComparable where T: IComparable)
{
    T first;
    T second;

    // This method would only exist if T was IComparable
    // The syntax and meaning is the same as for ordinary template 
    // methods except the class parameter is used.
    public int CompareTo(object other) where T: IComparable
    {
        int r1 = first.CompareTo(other.first);
        return r1 != 0 
            ? r1 
            : second.CompareTo(other.second);
    }

    //....
}