always allow unification for Expr type params (fixed issue #662)

type.ml

@@ -661,7 +661,7 @@ let rec type_eq param a b =
 		if e1 != e2 && not (param = EqCoreType && e1.e_path = e2.e_path) then error [cannot_unify a b];
 		List.iter2 (type_eq param) tl1 tl2
 	| TInst (c1,tl1) , TInst (c2,tl2) ->
-		if c1 != c2 && not (param = EqCoreType && c1.cl_path = c2.cl_path) then error [cannot_unify a b];
+		if c1 != c2 && not (param = EqCoreType && c1.cl_path = c2.cl_path) && (match c1.cl_kind, c2.cl_kind with KExpr _, KExpr _ -> false | _ -> true) then error [cannot_unify a b];
 		List.iter2 (type_eq param) tl1 tl2
 	| TFun (l1,r1) , TFun (l2,r2) when List.length l1 = List.length l2 ->
 		(try