Collaborative Container

[Salih Dincer]

Greetings to everyone who has fallen in love with D... 😀

I have a collaborative container (Node) and another convenience 
struct (S) where they work together. Node can be a struct or 
class, so a 3rd type is not allowed.

This is easy to do, `static if(is...` came to our rescue. But 
because the compiler interprets the alias first for some reason, 
we get this error message:

> source_file.d(13): Error: no property `CLL` for type `TestBP`
> source_file.d(55): Error: template instance `source.S!(TestBP)` 
> error instantiating

I implemented the solution using the mixin template and dividing 
S into 2 parts.  Moreover, I got rid of the `static assert`:

```d
struct S(Node)
{
   static if(is(Node == struct))
   {
     private Node * data;
   }
   else static if(is(Node == class))
   {
     private Node data;
   }
   else static assert(0, "\nIt's not a container!");

   alias T = typeof(Node.CLL);

   this(T value) { data = new Node(value); }
}

struct S_part1(Node)
if(is(Node == struct) || is(Node == class))
{
   mixin S_part2!Node;
   this(T value) { data = new Node(value); }
}

template S_part2(Node)
{
   static if(is(Node == struct))
   {
     private Node * data;
   }
   else static if(is(Node == class))
   {
     private Node data;
   }

   alias T = typeof(Node.CLL);
}

class Bar {
   int data;
   alias CLL = data;
   this(int i) { data = i; }
}

struct Foo {
   int data;
   alias CLL = data;
   this(int i) { data = i; }
}

void main()
{
   // By-Pass Test:
   enum TestBP { None }
   S!TestBP test1;

   // Test okay:
   enum TestOK { CLL }
   S!TestOK test2;

   // New solution:
   S_part1!Foo foo;
   S_part1!Bar bar;
}
```
SDB at 79