Delphi interface inheritance: Why can't I access ancestor interface's members?

Assume you have the following:

//Note the original example I posted didn't reproduce the problem so
//I created an clean example  
  type
    IParent = interface(IInterface)
    ['{85A340FA-D5E5-4F37-ABDD-A75A7B3B494C}']
      procedure DoSomething;
    end;

    IChild = interface(IParent)
    ['{15927C56-8CDA-4122-8ECB-920948027015}']
      procedure DoSomethingElse;
    end;

    TGrandParent = class(TInterfacedObject)
    end;

    TParent = class(TGrandParent)
    end;

    TChild = class(TParent, IChild)
    private
      FChildDelegate: IChild;
    public
      property ChildDelegate:IChild read FChildDelegate implements IChild;
    end;

    TChildDelegate = class(TInterfacedObject, IChild)
    public
      procedure DoSomething;
      procedure DoSomethingElse;
    end;

I would think that this would allow you to call DoSomething but this doesn't seem to be the case:

procedure CallDoSomething(Parent: TParent);
begin
  if Parent is TChild then
    TChild(Parent).DoSomething;
end;

Its clear that the compiler is enforcing the interface inheritance because neither class will compile unless the members of IParent are implemented. Despite this the compiler is unable to resolve members of the IParent when the class is instantiated and used.

I can work around this by explicitly including IParent in the class declaration of TMyClass:

TMyClass = class(TInterfacedObject, IChild, IParent)

Nevermind, this doesn't work around anything.