Narrowing one value based on another is not preserved across repeat conditionals

[JoshuaKGoldberg]

🔎 Search Terms

narrow union two values narrowing if condition

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwHMQMBlDGLVAgeRgFVVRFKRgAKASgC54BncygXgAfeGiYtgAbgBQM0JFgIU6bHnhwwILADcQAMRg4Atm0RHjPfhSrd4OnFmlyQADwAOOGBiRpMufAwQfjZjHFAeAHIoSJF4SIAjSI54AG8ZeHgwPH54VFgjAHcoBIgEAF5CYjIbGnpGEGZUVk5ZDPgsRHhQ8IryyoAiKAH4ADJR+ABCfJgikrKU9MzMuAxkGFRZTIBfOUyAen34ADpTvY6untB4fsHhxfaV8G09QxM2GbnSkA4t+F3tkA

💻 Code

declare function getStringOrUndefined(): string | undefined;

declare function receiveFrom(from: string): void;

export function test(mode: 'a' | 'b') {
  const narrowable = getStringOrUndefined();

  if (mode === "a" && !narrowable) {
    return;
  }

  // ...

  if (mode === "a") {
    receiveFrom(narrowable);
    //          ~~~~~~~~~~
    // Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
    // Type 'undefined' is not assignable to type 'string'.
  }
}

🙁 Actual behavior

narrowable is type string | undefined inside the second if.

🙂 Expected behavior

We know that if mode === "a" then narrowable must be truthy. Otherwise the early return; would have been hit.

Additional information about the issue

I have a hunch I've seen this before, but searched through issues a bunch and couldn't find anything...

This is a simplified example from real-world code. The // ... contains some logic that must happen regardless of mode.

[MartinJohns]

Duplicate of #52822.