Fix check of rhs of type family instances (#2157)

compiler/typecheck/TcMType.lhs

@@ -1681,8 +1681,7 @@ checkValidTypeInst typats rhs
        ; mapM_ checkTyFamFreeness typats
 
          -- the right-hand side is a tau type
-       ; checkTc (isTauTy rhs) $ 
-	   polyTyErr rhs
+       ; checkValidMonoType rhs
 
          -- we have a decidable instance unless otherwise permitted
        ; undecidable_ok <- doptM Opt_UndecidableInstances
@@ -1735,11 +1734,6 @@ tyFamInstInIndexErr ty
          colon) 4 $
       ppr ty
 
-polyTyErr :: Type -> SDoc
-polyTyErr ty 
-  = hang (ptext (sLit "Illegal polymorphic type in type instance") <> colon) 4 $
-      ppr ty
-
 famInstUndecErr :: Type -> SDoc -> SDoc
 famInstUndecErr ty msg 
   = sep [msg, 

compiler/typecheck/TcTyClsDecls.lhs

@@ -286,8 +286,7 @@ tcFamInstDecl1 (decl@TySynonym {tcdLName = L loc tc_name})
        ; t_typats <- mapM tcHsKindedType k_typats
        ; t_rhs    <- tcHsKindedType k_rhs
 
-         -- (3) check that 
-         --     - check the well-formedness of the instance
+         -- (3) check the well-formedness of the instance
        ; checkValidTypeInst t_typats t_rhs
 
          -- (4) construct representation tycon