rustc: Fail to unify if two ty_vars don't unify

Closes #500

src/comp/middle/ty.rs

@@ -1943,7 +1943,12 @@ mod unify {
                 alt (struct(cx.tcx, expected)) {
                     case (ty::ty_var(?expected_id)) {
                         auto expected_n = expected_id as uint;
-                        union(cx, expected_n, actual_n);
+                        alt(union(cx, expected_n, actual_n)) {
+                            case (unres_ok) { /* fall through */ }
+                            case (unres_err(?t_e)) {
+                                ret ures_err(t_e);
+                            }
+                        }
                     }
                     case (_) {
 

src/test/compile-fail/binop-typeck.rs

@@ -0,0 +1,9 @@
+// xfail-stage0
+// error-pattern:mismatched types
+// issue #500
+
+fn main() {
+  auto x = true;
+  auto y = 1;
+  auto z = x + y;
+}