Type mismatch

**TypeScript Version:** 

1.8.10

**Code**

``` ts
function f(a: { b?: string; c?: number; }){
    // something else
}
f({b: ""}); //  no error as intended
f({b: 1}); //  error as intended
f(1); // expected: error, actually: no error
f(""); // expected: error, actually: no error
```

**Expected behavior:**
see comments in the code.
**Actual behavior:** 
see comments in the code.
