Comment by ChadNauseam

The value returned by `getValue b` has type `pickType b`. It cannot be used in a way that depends on its type until you check the value of `b`. If `b = True`, then you know that `pickType b = pickType True = Nat` and that therefore `getValue b` has type `Nat`. There is no option to check the type of a value at runtime. You simply cannot use it in such a way as to create a runtime type error. (This is what it means for the types to be erased. The runtime doesn't know what type it's supposed to be!)