Switch on interface with single case not considered exhaustive

[mpal9000]

TypeScript Version: 2.5.2

Code

interface Action {
  readonly type: 'Action1',
}

export const assertNever = (action:never):never => {
  throw new Error('Unhandled action')
}

const fn = (action: Action) => {
  switch (action.type) {
    case 'Action1': {
      return 'action1'
    }

    default: {
      return assertNever(action) // incorrect error
    }
  }
}

Expected behavior:
action inside default block should have type never

Actual behavior:
action inside default block has type Action

Similar fixed issue: #12771

Playground: https://goo.gl/rs6TgW

[DanielRosenwasser]

Hey @mpal9000, this looks to me like a potential duplicate of #18056

[mpal9000]

Hi, you can close this one if you want. Checking action.type in assertNever() is not a solution, because it doesn't work for unions. A built in exhaustiveness checking in switch and future match, without manual assertions, would be an awesome feature in my opinion.