Why this compiles?

[Antonio via Digitalmars-d-learn]

In this example,  ```a``` and ```b``` vars are not of the same 
type and don't implement the same interface.  **why 
```assert(a==b)``` compiles?**

```d
interface IOpt(T) {
  bool opEquals(const IOpt!T) const @safe pure;
}

class None(T) : IOpt!T {
  bool opEquals(const IOpt!T other) const @safe pure => true;
}

void main() {
  auto a = new None!int();
  auto b = new None!string();

  assert(a==b);
}
```