Optional chaining control flow cannot narrow discriminated unions.

[whzx5byb]

Bug Report

🔎 Search Terms

optional chaining discriminated union control flow

🕗 Version & Regression Information

• This is the behavior in every version I tried.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Position {
  x: number;
  y: number;
  computed?: never;
}

interface RelativePosition {
  x: number | string;
  y: number | string;
  computed: Position;
}

declare var pos: Position | RelativePosition;

function getComputedX(pos: Position | RelativePosition): number {
  return pos.computed?.x ?? pos.x;
  // error here. Expect to be type "number", but actually "number | string";
}

if (pos.computed?.x !== undefined) {
  pos; // Expected to be type "RelativePosition", but actually "Position | RelativePosition"
  pos.computed; 
  // Inconsistence here. 
  // If "pos" is "Position | RelativePosition", then "pos.computed" should be "Position | undefined".
  // If "pos" is "RelativePosition", then "pos.computed" should be "Position". (And this is expected.)
  // But here "pos" is "Position | RelativePosition", but "pos.computed" is "Position".
  // The inconsistence make me think it is a bug instead of a suggestion.
}

Possibly related: #34597

[andrewbranch]

I think this is essentially a duplicate of #32399/#18758, but is notably not fixed by #38839 (it looks like maybe that’s limited to narrowing with typeof).

[andrewbranch]

The original repro in #18758 can be changed very slightly to show this bug:

type A = { type: "a" }

type X = { type: A, a: string }
type Y = { type?: undefined, b: string }

declare let x: X | Y

if (x.type?.type === "a") {
	x.a // Type Error
}

[ShuiRuTian]

Interesting. I remember I try to handle expression with optional chain, but it seems I do not do it well.
However, I would have a look at this.
Thanks a lot for the case @whzx5byb and analytics @andrewbranch !

[ShuiRuTian]

This is fixed in #38839 now!

1. add nest QuestionQuestionToken(??) support
2. undefined type could be narrowed correctly

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.