Destructured argument default value is not type checked

**TypeScript Version:**  2.4.1

**Code**

```ts
function test(n: number,
	{
		method = 'z'
	}: {
		method?: 'x' | 'y',
	} = {
		method: 'y'
	})
{
	console.log('test', n, method);
}

test(1);
test(2, {});
```

**Expected behavior:**

Compile error: `z` is not a possible value for `method`'s type.

**Actual behavior:**

Prints:
test 1 y
test 2 z