Type narrowing not preserved in closure between narrowing and usage before last assignment (incomplete "Preserved Narrowing in Closures Following Last Assignments") #57405

🔎 Search Terms

• Preserved Narrowing in Closures
• Last Assignments

Read #56908 and https://devblogs.microsoft.com/typescript/announcing-typescript-5-4-beta/#preserved-narrowing-in-closures-following-last-assignments because I feel like this related case should also be handled.

🕗 Version & Regression Information

• I was unable to test this on prior versions because it didn't exist at the time (new for 5.4.0-beta: https://devblogs.microsoft.com/typescript/announcing-typescript-5-4-beta/#preserved-narrowing-in-closures-following-last-assignments).
• This behaviour happens in the latest 5.0.4-beta and nightly.

I'm not sure if this is a bug, intended behaviour or a missing feature. To me, this looks like incomplete type narrowing in closures before the last assignment, when it should be trivial to narrow the type.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20240214#code/GYVwdgxgLglg9mABCAzgUwHIgLYCM0BOAFAB4BciYO+BAlIgN4BQiiBaUIBSUBIaTAL5MmoSLASIoaFFADqcAgGsYYAOZFgcOLgCGBClTyFEAH2RgAJmmCq0l+s1YAbDoggALNBCX2AYtp6BpTUJubg1rZg9izuCLKIwEgAvIhE9MkAfIyxrBDxUIgwKFjGBIipnt6+lgE6+ogAhMmpETZ2lrlxYAlQ2AAOFUUloeUAZGPI6KU0RFU+-oH6tCKsrAD064gAksBSHsWIznbDcdjYaGDSlohwIFAANPtoiIQEiohqcDKIugDuugAnl1NvManUgkMtPUCKtEMJWOxONxEmAhCIxNB4DwZFAAELvXxgTRLYJGGhmCyRDqOWKuQpgxYwwyjSltKIxPIFVFDdIVbJONb5HqFYozEyVLwLWqkpotKntaKdNbdXoDIZi1kTKaYUZzKXg0krEFbXb7Q7HaKnfLnS7XW73J5QLyvAjvcpfH7-IFdRkymFQ0lwhFsDhcJBJIRAA

💻 Code

function useNumber(x: number) {
  return true
}

function testBroken(foobar: number | undefined) {
  let checkedFoobar: number | undefined
  const fn = () => {
    const isNumber = checkedFoobar !== undefined
    const tmp = isNumber && useNumber(checkedFoobar) // error; because `checkedFoobar` is `number | undefined`

    // If this line is commented out, the error goes away in 5.4.0
    // (Before 5.4.0 the error always exists, even without this assignment)
    checkedFoobar = foobar

  }
  return fn
}

🙁 Actual behavior

In the sample above:

    const isNumber = checkedFoobar !== undefined
    const tmp = isNumber && useNumber(checkedFoobar) // error; because `checkedFoobar` is `number | undefined`

🙂 Expected behavior

TypeScript should know that checkedFoobar is number (and keep that knowledge in the isNumber variable).
There's no assignment between the check and the usage, so any type narrowing should hold.

In the sample above:

    const isNumber = checkedFoobar !== undefined
    const tmp = isNumber && useNumber(checkedFoobar) // works; because `checkedFoobar` should be `number`

Additional information about the issue

An ugly workaround is to repeat the undefined check directly in the condition:

const tmp = checkedFoobar !== undefined && useNumber(checkedFoobar) // works; because `checkedFoobar` is `number`