opDispatch and UFCS

[John Colvin via Digitalmars-d-learn]

struct S
{
    int a;
    template opDispatch(string s)
    {
        template opDispatch(T...)
        {
            auto ref opDispatch(Args ...)(auto ref Args args)
            {
                return S(mixin(`a.` ~ s ~ (T.length ? `!T` : ``) 
~ `(args)`));
            }
        }
    }
}
int foo(int a, int b)
{
    return a + b;
}

S bar(S s, int b)
{
    return S(s.a - b);
}

unittest
{
    auto s = S(3);
    s = s.bar(1);
    assert(s.a == 2);
    s = s.foo(1);
    assert(s.a == 3);
}

this gives me:

test.d-mixin-10(10): Error: function opdispatchtest.bar (S s, int 
b) is not callable using argument types (int, int)
test.d(29): Error: template instance 
test.S.opDispatch!"bar".opDispatch!().opDispatch!int error 
instantiating

Bug? Or am I misunderstanding how these two features are supposed 
to interact?