Object destructuring breaks disjoint union type.

[AndrewSouthpaw]

As an example:

type Props =
  | { type: 'foo', item: { id: number, parentId: number | null } }
  | { type: 'bar', item: { id: number, parentId: number } }

function test(props: Props) {
  // This works
  const yay = props.type === 'foo'
    ? props.item.id
    : `/parents/${props.item.parentId}/items/${props.item.id}`

  // This does not
  const { type, item } = props
  const boo = type === 'foo'
    ? item.id
    : `/parents/${item.parentId}/items/${item.id}`
}

I'm running v0.46. Curiously, the try flow REPL doesn't exhibit an error, I'm only getting it locally.

Any ideas?

[vkurchatkin]

This shouldn't really work. When you destructure a union, types get flattened.

[AndrewSouthpaw]

Hmm... I guess that makes sense. Thanks for pointing that out. I imagine it would be hard/impossible to keep the union with destructuring?

[iddan]

@vkurchatkin Why is that?

[villesau]

Some related issues:
#6146
#6805
#6594
#6408
#5745
#4772