Debugging typically helps you gain insight in what's happening down in the code. Debugging the type-system is a special challenge, though. The MPS type-system engine performs type calculations on the type-system rules trying to match types to the language elements used in the code. You, as a language designer, are interested in seeing the high-level picture of what's happening, what type is assigned to what element, when and why.
The MPS Type-System Trace tool, that some of you may already know, gives you exactly that.
We've prepared a Typesystem Debugging document describing the ways you can