undefinedTy  :: Type   -- See Note [Error and friends have an "open-tyvar" forall]
undefinedTy  = mkSigmaTy [openAlphaTyVar] [] openAlphaTy
undefinedTy  :: Type
undefinedTy  = mkSigmaTy [openAlphaTyVar] [] openAlphaTy
undefinedTy  = mkSigmaTy [openAlphaTyVar] [] openAlphaTy