Why isn't this supported in Unity?

public class Container<T>
{
    private static class PerType<U> where U : T
    {
        public static U item;
    }

    public U Get<U>() where U : T
    {
        return PerType<U>.item;
    }

    public void Set<U>(U newItem) where U : T
    {
        PerType<U>.item = newItem;
    }
}

I'm getting this error:

error CS0305: Using the generic type Container.PerType' requires2' type argument(s)