Type <generic> cannot be used to index type <array>

**TypeScript Version:**
0 2.3.1-insiders.20170416 (Playground)

**Code**
```ts
type X<I extends number> = ['a'][I];
```

**Expected behavior:**
No error

**Actual behavior:**
`Type 'I' cannot be used to index type '["a"]'.`

I thought this may have worked before. Potential regression?
