when using rest operator flow requires optional object members to be returned.

[acornejo]

I have an object with an optional property. I have a function which receives an instance of said object and returns another instance whithout the optional property. Flow does not seem happy with this.

Example that reproduces issue:

https://flow.org/try/#0C4TwDgpgBAYg9nKBeKBvAUFKBDAXFAZ2ACcBLAOwHMAaTKAIwH59yBXAW3omNoF910AM1bkAxsFJxyUEtnIFBcYuwAUiuPnhwAlJoRo6oqUTT1qUAHRXiEIr2RR1Abjo3grYtJtEXvIA

type Foo = {
  a: string,
  b?: number,
}

function transform(foo: Foo): Foo {
  const {b, ...rest} = foo;
  return rest;
}

[wchargin]

Closely related to #6863. Using return {...rest} suffices. Yes, Flow should be able to infer this type at rest-time.