Regression in 6.0: type inferred from const with broader type is too narrow

[trevorade]

🔎 Search Terms

"infer", "regression", "wide", "narrow"

🕗 Version & Regression Information

• This changed between versions 5.9.3 and 6.0.2
• This changed in commit or PR ???
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about YES
• I was unable to test this on prior versions because N/A

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.2#code/C4TwDgpgBAggQlAvFA5DFUA+q4oNwBQBAxgPYB2AzsFACICiAYjAKoAyAKgFywLJr4ixADYBDSpSgBhKAG8CUOkjpNWnQgqgBbCMAAWpACYAKAJRzNiygHcAlsGJ7j+25QB0tc-MU+oxcdACPABGAE4QogDWhL6K-pSBuCHhUTE+AL6amelAA

💻 Code

type AB = 'A' | 'B';

const DEFAULT: AB = 'A';

class C {
  D = DEFAULT;

  method() {
    switch(this.D) {
      case 'A': break;
      case 'B': break;
    }
  }
}

🙁 Actual behavior

C.D infers to "A"

🙂 Expected behavior

C.D infers to AB

Additional information about the issue

Somewhat common occurrence in Angular code.

Hundreds of instances of this class of error in Google.

[RyanCavanaugh]

Bisects to #62243, which is pretty surprising?

[trevorade]

Thanks for fixing this!

@RyanCavanaugh, any timeline for tsc 6.0.3?

[RyanCavanaugh]

Likely in the next week or so, we're going to want to see if any other patch-worthy bugs come in