Optional chaining type narrowing on parenthesized assignment expression

[mjy9088]

🔎 Search Terms

optional chaining type narrowing

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about common-bugs-that-arent-bugs

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBA8mwEsD2A7KBeKBvKpIC4oAiAZSQFsIiAaKANwEMAbAVwkJRfICMIAnKAF8oAH2y5w7YgDlUVIQG4AUEoAmEAMZMGfaADMWKDYlRQAzhQhwTKABQBKQteQplSg0ZtQA7gmAALJBZgAFUUdT0EFAhVBw4uXgExQwiomOwlKCgmCGAoJHgXJ0LUNyyAenKfJD4AazNMqAQ9KFtbAq9MC0pnVAd7ADo8aHRRqAByMkpx+wysrN1gFj40DpcBxlYIZSzBFQXc5bQUiEjo1WU9tU1tXSgPYxdzS16UZle42BK0ZPDTtIuKgeXjAfCQ3By5AYiA0n04PH4oigJzO6SwjRyeTWqGKXl+qXOZSglSgKCQeW8NXqjWarXa3wwzx633e336AH4hpIMGNJpYZnN5osjvlvhtmGwdkJ9sSqpS6g0sti0F0XiymB97FLafSbJzhjzMHzprN0fMoMKVqKbOKtlKrgcllaUQDLkogA

💻 Code

type Option = { type: "Some", value: number } | { type: "None" };

declare function someOption(): Option;

function withoutUndefined(): number | undefined {
  let option: Option;

  // works
  if ((option = someOption()).type === 'Some') {
    return option.value;
  }

  return undefined;
}

declare function someOptionalOption(): Option | undefined;

function problematic(): number | undefined {
  let option: Option | undefined;

  // not works
  if ((option = someOptionalOption())?.type === 'Some') {
    return option.value;
  }

  // works
  option = someOptionalOption();
  if (option?.type === 'Some') {
    return option.value;
  }

  return undefined;
}

🙁 Actual behavior

Error 'option' is possibly 'undefined'. occurs

🙂 Expected behavior

No error

Additional information about the issue

No response

[Andarist]

Duplicate of #56998