add a test where mode inference ought to fail

src/test/compile-fail/mode-inference-fail.rs

@@ -0,0 +1,11 @@
+// In this test, the mode gets inferred to ++ due to the apply_int(),
+// but then we get a failure in the generic apply().
+
+fn apply<A>(f: fn(A) -> A, a: A) -> A { f(a) }
+fn apply_int(f: fn(int) -> int, a: int) -> int { f(a) }
+
+fn main() {
+    let f = {|i| i};
+    assert apply_int(f, 2) == 2;
+    assert apply(f, 2) == 2; //! ERROR expected argument mode ++
+}