Unboxed sums are not Typeable
While the Typeable machinery introduce in the type-indexed Typeable rework (#11011 (closed)) is fully capable of representing unboxed sums, we currently don't allow this since it would require that we generate an enormous number of bindings in
GHC.Types. This is because we need to generate both
KindRep bindings for each type constructor and its promoted data constructors.