Type parameter should not be assumed truthy

**TypeScript Version:**  3.1.0-dev.20180912

**Code**

```ts
function f<T extends string | undefined>(x: T): number {
    return x && 42;
}
f(undefined).toFixed();
```

**Expected behavior:**

Error -- the output may be `""` or `undefined`.

**Actual behavior:**

No error. (Error at runtime.)