No implicit conversion due to several general restrictions

Building a piece of C # code using generics I created code that looks like this:

public class Test<U,V,W>
    where U : V
    where V : W
{
    public W Cast(U argument)
    {
        return argument;
    }
}

      

        
        
        
      

    

The string return

cannot compile (in mono 3.2.8.0: "Cannot implicitly convert the type of U to W") although it seems clear that U must be a subclass of W (technically there can be value types but the above fails even if all three common parameters are limited reference types). If U

limited directly from W

, everything works fine.

The above code is trivial to fix (just insert the cast into V, which, of course, will always be successful). I have two questions, most importantly, why is the code failing to compile? Links to the relevant C # spec snippets (if any) would be most welcome! Second, would the (seemingly redundant) conversion to V

introduce (useless) runtime type checking?