Destructuring breaks tagged union case inference

[bcherny]

TypeScript Version: 2.8.3

Search Terms: union, destructure

Code

type Message =
    { type: 'sendMessageToThread', data: { threadId: number, message: string }}
  | { type: 'createThread', data: { userIds: number[] }}
  | { type: 'addUserToThread', data: { threadId: number, userId: number }}

// Good case
function a(message: Message) {
  switch (message.type) {
    case 'sendMessageToThread':
      console.log(message.data.threadId)
  }
}

// Bad case
function b({ type, data }: Message) {
  switch (type) {
    case 'sendMessageToThread':
      // Error: Property 'threadId' does not exist on type
      console.log(data.threadId)
  }
}

Expected behavior:

TS should infer which case the tagged union falls into regardless of whether or not the parameter is destructured. data should be inferred as { threadId: number, message: string }.

Actual behavior:

data is inferred as the union of all data values in Message.

Playground Link

[mhegazy]

Duplicate of #14630, and #15482

[bcherny]

Thanks for digging up those issues @mhegazy. This seems like a real issue, so I'm not sure why those were closed out. Maybe it's worth keeping one of them open to track the issue? This is pretty surprising behavior from a user's point of view.

[mhegazy]

I would merge this in #9998. these are design limitation for how control flow analysis is implemented.

[bcherny]

Sounds good to me.