interface inference

[Antonio]

```d
interface I {
         bool check();
}
class A : I {
         bool check() =>true;
}
class B : I {
         bool check() =>false;
}

I aOrB(bool check) => check ? new A() : new B();

void main()
{
   assert( aOrB(true).check );
}
```

Compiler error:

```d
x.d(11): Error: cannot implicitly convert expression `check ? new 
A : new B` of type `object.Object` to `x.I`
```

Basically, the ternary conditional ```?:``` result type is not 
inferred even if the type returned by the two possibilities are 
the same.

**Is it a bug or the expected behaviour?**