Conditional type not narrowed by control flow

[PaperStrike]

Bug Report

🔎 Search Terms

conditional guard, conditional flow

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about conditional types.

⏯ Playground Link

Playground link with relevant code

💻 Code

type PackageForFunctions<T> = T extends Function ? (required: true) => T : T;

const getOriginal = <T>(value: PackageForFunctions<T>): T => {
  if (typeof value === 'function') return value(false);
  return value;
};

🙁 Actual behavior

• No error on value(false). Wrong for value can only be a function when it is (required: true) => T.
• Error "not assignable to type 'T'" in the last return. Wrong for value must be T if it is not a function.

🙂 Expected behavior

• Throw an error as the required param is true on value(false).
• No error in the last return as it must be T at that line.

[jcalz]

Duplicate of #33912