compile time value addition. Won't compile, but I'm not sure why.

errors:

d_playground.d(38): Error: pure nested function 'a' cannot access mutable data 'this'
d_playground.d(38): Error: pure nested function 'a' cannot access mutable data 'this'
d_playground.d(14): Error: template instance d_playground.value.opBinary!("+", value).opBinary.intermediateValue!(plus, this, rhs) error instantiating
d_playground.d(47):        instantiated from here: opBinary!("+", value)
d_playground.d(47): Error: template instance d_playground.value.opBinary!("+", value) error instantiating

playground.d

struct value
{
    int a;


    //this(Op, A, B)( in intermediateValue!(Op, A, B) i) {
        //a = i.a;
    //}
    //this( int b ) { a = b; }

    const auto
        opBinary(string op, T)(in T rhs) const pure {
        static if (op == "+")
            return intermediateValue!(value.plus,this,rhs)();
    }

    ref value opAssign(T)( in T t ) {
        a = t.a;
        return this;
    }

    static
    int plus(T1, T2)(in T1 x, in T2 y) pure {
        return x.a + y.a;
    }

}

struct intermediateValue(alias Op, alias A, alias B)
{

    auto opBinary(string op, T)(in T rhs) const pure {
        static if (op == "+")
            return intermediateValue!(value.plus,this,rhs)();
    }

    @property auto a() const pure {
        return Op(A, B);
    }

}

void test() {
    value a = value(2);
    value b = value(3);
    value c;
    c = a + b;
}


void main()
{
    test();
}