No polymorphism?

[Dgame via Digitalmars-d-learn]

I may be just tired, but could somebody explain this behaviour to 
me? It seems odd to me:

----
interface I
{
}
class S : I
{
}

void test1(I[])
{
}

void test2(I)
{
}

void main()
{
        test1([new S()]); // Works
        test2(new S()); // Works
        
        I i = new S();
        test2(i); // Works
        
        S s = new S();
        test2(s); // Works
        
        I[] si = [new S()];
        test1(si); // Works
        
        S[] ss = [new S()];
        test1(ss); // Fails
}
----

Compiler output: test.d(32): Error: function test1 (I[] _param_0) 
is not callable using argument types (S[])

Why isn't the compiler able to deduce S[] => I[]? Or is it just 
me?
I've tried dmd 2.075