Support covariant return types. Specifically, allow an overriding method to have a more derived reference type than the method it overrides. This would apply to methods and properties, and be supported in classes and interfaces. This is one possible alternative to a this type proposed in #311.

This would be useful in the factory pattern. For example, in the Roslyn code base we would have

class Compilation ...
{
    virtual Compilation WithOptions(Options options)...
}

class CSharpCompilation : Compilation
{
    override CSharpCompilation WithOptions(Options options)...
}

The implementation of this would be for the compiler to emit the overriding method as a "new" virtual method that hides the base class method, along with a bridge method that implements the base class method with a call to the derived class method.