variable incorrectly typed as true instead of boolean when change is made inside IIFE

[mdouglass]

🔎 Search Terms

type inference boolean

🕗 Version & Regression Information

TypeScript 5.4.5

⏯ Playground Link

https://typescript-eslint.io/play/#ts=5.4.3&fileType=.ts&code=IYZwngdgxgBAZgV2gFwJYHsIwLbFRACgEoAuGABQCd1tUQBTAHgDd1UATAPhgG8AoGDAA29ZDGbAhCejAC8MZJWkCYwAO54xBUJFjE53foMESpM%2BXEkMVAXyLEVqODAKnpRXisFRMIdCIA6IXQAcwIAcjcZOgUlenCiWxh6IQZPYxgfCD9A4LDIyWkYGMtU%2BMTBGz4qvlYOHDxCRKA&eslintrc=N4KABGBECmAeAu0B2ATAzpAXGA2jNANgJZLyYBO0AxgPYC2dyK0KkANFAA4ECuA5iUwABeAE9O0NFXJFO8ALSTipAPRp4MqgrET5VABbUA1i0gBdEAF8QQA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

💻 Code

async function main(): Promise<void> {
  let value = true
  await (async () => {
    value = false
  })()
  if (value) {
    console.log('value is true')
  } else {
    console.log('value is false')
  }
}

void main()

🙁 Actual behavior

I originally reported this as a bug in typescript-eslint but was told that the underlying type deduction was incorrect in this case and therefore it wasn't a bug there.

The variable value in this code is deduced as type true (seen via hover in vscode on value in the if statement) but would be correctly typed as boolean since it can be (and in this code sample is) changed. If you run the code it logs value is false despite the type being true.

I have a slightly simpler playground that does not involve an async function here: https://www.typescriptlang.org/play/?ts=5.4.5#code/DYUwLgBAbghsCuIIF4IDM4GcQG4BQA2gORoD2pRAugHRkBOAojAMYAWAFO2CAB5gCUKAHwQA3ngjQ4iFBDB1E+AL798ASzQR2sBCEHjJzUgDtMpUNWCkA5uyI6ZazHIUgiqvEoghg2MRIgjU3MQSxs7ByQndCw3DyUgA

🙂 Expected behavior

value should have the type boolean

Additional information about the issue

xref typescript-eslint/typescript-eslint#9055

[mdouglass]

clsoing after pointed at #9998