closures + struct: Error: forward reference to inferred return type of function call

[ddcovery]

In this example, I try to use D Voldemore (fantastic) 
functionallity to implement a pattern usually used in javascript.

Basically, a function closure represents the state, and the 
methods to access this state are returned in the form of union 
(union has not state itself).

void main(){
   assert(number(10).next.value==11 );
   assert(zero()==number(0) );
   assert(zero()==number(1).previous );
   auto ten = number(10);
   auto nine = ten.previous;
   assert(nine == number(9));
}

auto number(int _value=0){
   struct Number {
     int value() {
       return _value;
     };
     Number next() {
       return number(_value+1);
     };
     Number previous(){
       return number(_value-1);
     };
     int opCmp(Number other){
       return _value - other.value;
     };
   }

   return Number();
}

auto zero() {
   return number(0);
}

Problem is that Union methods needs to call it's containing 
function and compiler doesn't accept it:

main.d(16): Error: forward reference to inferred return type of 
function call number(_value + 1)
main.d(19): Error: forward reference to inferred return type of 
function call number(_value - 1)

I know there is another ways to solve this problem easily 
(without closures), but I was really curious about if it was 
possible.