I want to implement a generic method on a generic class which would allow to cast safely, see example:

```
public class Foo<T> : IEnumerable<T>
{
    ...
    public IEnumerable<R> SafeCast<R>()
        where T : R
    {
        return this.Select(item => (R)item);
    }
}

```

However, the compiler tells me that `Foo<T>.SafeCast<R>() does not define parameter 'T'`. I understand this message that I cannot specify a constraint on `T` in the method since it is not defined in the method. But how can I specify an inverse constraint?