Inferred return types

[Martin Nowak]

> class Foo
> {
>      auto foo ()
>      {
>          return "Foo";
>      }
> }
>
> class Bar : Foo
> {
>      auto foo ()
>      {
>          return "Bar";
>      }
> }
>
Ouch, what a terrible idea to base a class hierachy on inference.
But nonetheless covariance checking should be performed after inference.