[Bug?] Failed to type check identity function

```java
    @Test
    public void test_0x08A() throws IOException {
        String input = "fn id(mut x : []&'a mut int) -> &'a mut int { *x }";
        input += " { let mut v1 = 1; let mut f1 = &mut v1; let mut a = box f1; let mut p = id(a); !*p }";
        check(input, One);
    }
```
Why would this test fail to "line 1: incompatible argument(s)"?