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Destructuring breaks tagged union case inference #23613
Description
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.
Activity
mhegazy commentedon Apr 23, 2018
Duplicate of #14630, and #15482
bcherny commentedon Apr 23, 2018
Thanks for digging up those issues Mohamed Hegazy (@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 commentedon Apr 23, 2018
I would merge this in #9998. these are design limitation for how control flow analysis is implemented.
bcherny commentedon Apr 28, 2018
Sounds good to me.