Co(ntra)variantly (In)valid — while (!dead) learn();

Sometimes I have a problem and I think to myself, "I'll solve this with generic constraints and variance."

Now I have an

ISolutionFactory<in TProblem, out TSolution>
where TProblem : class, IProblem
where TSolution : class, ISolution

...but no one can figure out how to use it without getting "Invalid variance" errors from the compiler.