fix(partition): add exclusion when using type guard

[Boudewijn26]

Description:
Add exclusion to partition when using a type guard.

Previously support for a user defined type guard was added to partition. This meant that Typescript could understand the first item in the resulting tuple to match a certain type. What was missing was Typescript understanding the second item in the tuple to not be of that type. As an example:

type AMessage = { a: number };
type BMessage = { b: number };
type Message = AMessage | BMessage;
const aMessage: AMessage = { a: 1 };
const bMessage: BMessage = { b: 2 };
const message$: Observable<Message> = from([aMessage, bMessage]);

function isAMessage(msg: Message): is AMessage { 
  return msg.a;
}
const [aMessage$, bMessage$] = partition(message$, isAMessage);
bMessage$.subscribe((msg) => console.log(msg.b)); // no longer fails with this change

Without this change Typescript would consider bMessage$ to be the same type as message$: Observable<AMessage | BMessage>. Whereas we are sure bMessage$ only emits BMessage, because all AMessage will be put in aMessage$.

[cartant]

LGTM. Thanks.

[Boudewijn26]

Thank you for the quick response, happy to help!