Multiple cases inside of a switch statement not narrowing the type

[boblauer]

Bug Report

If you have a switch statement that has multiple cases that fall through, and those case statements are not first, TypeScript will not narrow your types in those statements.

It's a bit confusing to explain, but if you look at the playground link below, hopefully it makes sense. You can see if that you get rid of the first case statement, everything works just fine.

🔎 Search Terms

switch, case

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

type Thing =
  | {
      type: 'a';
      onlyA: string;
    }
  | {
      type: 'b' | 'B';
      onlyB: string;
    };

const thing: Thing = {} as Thing;

switch (true) {
  case thing.type === 'B':
    break;

  case thing.type === 'b' && thing.onlyB === 'a': // TS error on this line
  case thing.type === 'a' && thing.onlyA === 'b': // and on this line
  case thing.type === 'a' && thing.onlyA === 'c':
    break;

  default:
    break;
}

🙁 Actual behavior

In the first 2 of the 3 case statements that are grouped together, TS will complain that onlyB and onlyA does not exist on type Thing. If you get rid of the very first case statement in the switch, the errors go away.

🙂 Expected behavior

I would expect this code to work as-is, without the need for the 3 case statements to be listed first in the switch statement.

[whzx5byb]

Related #37178 #8934

[boblauer]

The one big difference between those issues and this one (as far as I can tell) is that in my example, the 3 grouped case statements do work if they're the first case statements in the switch:

https://www.typescriptlang.org/play?ts=4.3.0-dev.20210331#code/C4TwDgpgBAKgFgSwHYHMoF4BQUoB8oDe2OJokAXFAOQCGVA3MSVAPZIA2IAgpQM7AAnZCkbMAvsXxFmOMhEpUARlTzUAQgyYk2nNX0HDRJMY0wBjNvyjBEqSvGEZCYqDV6xbIzJl4B3BMBmcFAAFIIArhAAlITEZm7QNsIAdHIY6OjUylAAZDnWnsk6IGrpmbRU5HEJBSlpGeV0uflJqEUc3GVZldW8iYX1GdRNebVtxVxdVGY9zIoCEDQA1qY48X1jKKng0A3qsyTziyveOAAmEABmNOHswFVzC8uMYkA

and if they are all listed as individual case statements, they also work:

https://www.typescriptlang.org/play?ts=4.3.0-dev.20210331#code/C4TwDgpgBAKgFgSwHYHMoF4BQUoB8oDe2OJokAXFAOQCGVA3MSVAPZIA2IAgpQM7AAnZCkbMAvsXxFmOMhEpUARlTzUAQgyYk2nNX0HDRJMY0wBjNvyjBEqSvGEZCYqDV6xbIzJl4B3BMBmcFAAFIIArhAAlITEZm7QNsIAdHIY6OjqVORaigIQNADWpjjxvImeqeDQGZlKKgBkDdaVOiBq6XV0Ocx5BcXepQktKWm11HRQTSOoyW1cndTKPSR9RSVQZRWj1Yu0jc1Js-N7Ztm5+euDUAAmEABmNOHswCs4awNiQA

So this type of narrowing is supported by TS inside of a switch(true) statement, but for some reason the order and grouping of the case statements makes the difference here.

[mhluongo]

Any progress here? Small but unfortunate bug for heavy union users