Not reporting error for incompatible generics in ternary

Paste the following code into https://flow.org/try/

```
// @flow

type A = { aThings: number }
type B = { bStuff: string }

type ObjectMap = { [name: string]: Value }

type Action = {
type: 'SOME_REDUX_ACTION',
payload: ObjectMap
}

function reducer (state = {}, action: Action): ObjectMap {
// Expect error: ObjectMap is not ObjectMap
return action.type === 'SOME_REDUX_ACTION' ? action.payload : state
}
```

The code contains a glaring error (returning `ObjectMap` where `ObjectMap` is expected), but Flow does not catch the error. I have tried make the test case as minimal as possible.