Templates for instantiating derived class

[rjkilpatrick]

Essentially, I would like to write a template that calls the 
constructor of the parent class or the constructor of the 
inherited class, depending on its type.

```d
#!/usr/bin/env dub
/+ dub.sdl:
	name "mwe"
+/
class Super {
     private int _a;
     this(){}
     this(int a) {
         _a = a;
     }

     auto opBinary(string op)(int rhs) const if (op == "+") {
         return new Super(_a + rhs); // Creates of type Super even 
when called from derived class
     }
}

class Derived : Super {
     this(){}
     this(int a) {
         _a = a + 1;
     }
}

void main() {
     import std : writeln;

     Super foo = new Super(1);
     Super foo2 = foo + 1; // Works fine as calls `Super` 
constructor

     Derived bar = new Derived(2);
     Derived bar2 = bar + 1; // Cannot implicitly cast `Super` to 
`Derived`
}
```

Some kind of `return new this(...)` would be good, but that's not 
possible.
I think it has to be done with templates, but I'm not sure how to 
do this.

Any help would be greatly appreciated.