Value can change "during" await, but compiler doesn't seem to know that

[EvanHahn]

Bug Report

I'm receiving the following error...

This condition will always return 'false' since the types '"b"' and '"a"' have no overlap.(2367)

...but the value might be "a". The condition might not return false. It's worth looking at the code snippets below, as I think they describe the issue better.

🔎 Search Terms

• TS2367
• Condition always returns false
• Value changes during await

🕗 Version & Regression Information

This does not seem to be a recent regression. When I test in the playground, I see this problem in all v4 versions.

⏯ Playground Link

Playground link with relevant code

💻 Code

let state: 'a' | 'b' = 'a';

async function a() {
    state = 'b';

    await Promise.resolve();

    // The following line has an error that I think should not happen:
    if (state === 'a') {
        throw new Error('hello');
    }
}

a()
state = 'a';

Here's another example that I think better isolates the issue:

let state: 'a' | 'b' = 'a';

async function a() {
    state = 'b';

    await Promise.resolve();

    const newState: 'a' | 'b' = state;
    console.log(newState); // <- type seems incorrect here
}

a()
state = 'a';

🙁 Actual behavior

Got TS2367.

🙂 Expected behavior

There is no error.

[jcalz]

We got another #9998, folks!

[fatcerberus]

Just want to point out that this is not specific to await; any function call might change non-local state.

[EvanHahn]

Maybe I should close this as a duplicate of #9998?

[EvanHahn]

Looks like many similar issues were closed as duplicates of #9998. I'll close this one, too.