TS complains about assignability instead of narrowing

[RedCMD]

🔎 Search Terms

type narrow

🕗 Version & Regression Information

since forever

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250322#code/MYewdgzgLgBANgS2jAvDA2gcgIKYDQyYBC+hAwpgLowCGEMokUA3AFCsBmArmMFAuFoAjYAApgACxoAnAFyFcMAD6ESy8pnWYAIpgCUMAN6sYMBBxijE0AHQJecLgBMAphHFTpeg4ZgB6PxgXaWkQaXoJYJcTUxhpFyguaTA2UwBfdlNzS0kZVDRifSMYNP9AiAkQLjgnIJCwmEj4mJiAmCcQGAgQAFsEiXsAcxgAdwQoCQZPVgygA

💻 Code

const list = ['A', 'B', 'C'] as const;

function abc(char: 'A' | 'B' | 'C' | 'D') {
  if (list.includes(char)) { // errors here
    return;
  }

  if (char == 'B') { } // should error here
  
  // do something with char
}

🙁 Actual behavior

TS complains about assignability

Argument of type 'string' is not assignable to parameter of type '"A" | "B" | "C"'.ts(2345)

🙂 Expected behavior

for TS to recognize that I'm not trying to assign types in the wrong places

and instead to detect that the following comparison appears to be unintentional if (value == 'B') { }

I want it to act like this:

function abc(char: 'A' | 'B' | 'C' | 'D') {
  if (char == 'A' || char == 'B' || char == 'C') {
    return;
  }


  if (char == 'B') { }

  // do something with char
}

Additional information about the issue

I'm trying to add an early exit at the start of the function
and I want the type system to reflect that char will no longer be 'A', 'B', 'C'
but instead it just tries to insult me

[MartinJohns]

Duplicate of #60073 and many many more.

instead it just tries to insult me

It's not trying to insult you, it's informing you of an error. Don't take compiler errors personally.