Usage of ad-hoc interfaces in C#

example.cs

interface Addable<T> {
    T AddTo(T rhs);
}

T AddTwoThings(T lhs, T rhs) where T : Addable<T> {
    return lhs.AddTo(rhs);
}

static class IntExtensions {
    // Pseudo-syntax
    int Addable<int>.AddTo(this int lhs, int rhs) {
        return lhs + rhs;
    }
}

static class FloatExtensions {
    float Addable<float>.Addto(this float lhs, float rhs) {
        return lhs + rhs;
    }
}

// …

int x = AddTwoThings(1, 2);
float y = AddTwoThings(1.3, 4.3);
var z = AddTwoThings(1, 3.4); // Doesn't compile!

// If `AddTwoThings` had been declared as
//
//  Addable AddTwoThings(Addable a, Addable b)
//
// then the last line would have compiled (we didn't want that!).