Optional chaining does not narrow types in the `else` branch

[kdmadej]

Bug Report

🔎 Search Terms

• optional chaining
• optional chaining narrow
• optional chaining discriminated

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about:
  • optional chaining
  • type narrowing

Tested on all TS versions available in the playground that support optional chaining.
At the time of writing:

• 4.2.0-beta
• 4.1.3
• 4.0.5
• 3.9.7
• 3.8.3
• 3.7.5

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = { type: 'A', a: 'a' }
type B = { type: 'B', b: 'b' }

declare const c: A | B | null
// declare const c: A | B | undefined
// declare const c: A | B // works fine for non-nullable unions

if (c?.type === 'A') {
    c; // narrowed to `A` as expected
} else {
    c; // `A` still present in the type
}

🙁 Actual behavior

In the second branch of the conditional statement the union type inferred for c still includes A. Same thing happens with ternary operator, functions with early returns, etc.

🙂 Expected behavior

In the second branch the type should be narrowed down by excluding A.

[kdmadej]

Might actually be related to #42404

[DanielRosenwasser]

I don't think they're related but yes, it looks like the expected type should indeed be null | B in the example above.

[ShuiRuTian]

@andrewbranch has almost already given the solution in #42450.

Only need to change TypeFlags.Undefined to TypeFlags.Nullable, I guess.

[kdmadej]

Yup, this issue is a duplicate of #40397
Sorry for the spam (not sure how I missed that in my search 🤦 ).

[ShuiRuTian]

Oh, so sorry for the misleading!
I found that the two issues are very similar, but not same: that issue is undefined, and this is null.
Since the PR to fix that issue has been merged(which only handle undefined condition, you could see more in the PR link), you could test in beta version(not 4.2.0-beta, it should end with a date) in playground.

So, I guess this issue would better to be reopened?

[kdmadej]

Indeed, | undefined works as expected in the nightly build but | null does not.

BTW I'd be more than happy to help out with a PR but I'd need some guidance as to where to start 😅 (i.e. is the aforementioned PR a good example, since it tackles a similar issue?)

[andrewbranch]

I think this is going to be a one-identifier change on top of my recent PR; will take a look this afternoon. Thanks for offering @kdmadej, but if I made a simple mistake I want to go back to it and make sure I understand why. Feel free to look at issues labeled "help wanted" or "good first issue" though!