In Blender's start up scene I switched to ProRender than added a plane.
I gave both the plane and the cube the default shader and the default Sun&Sky.
When I try the viewport render the cube casts a shadow the cube it selfs seems to be missing.
From a different angle:
Same thing happens when I try to render to image:
My hardware and software: I, using Radeon7970
I attached Blender's console and the Trace Dump.