Narrowing types by `typeof` inside `switch (true)` does not work

TypeScript Version: 3.8.3 Nightly

Search Terms:

• switch statement
• switch (true)

Code

let x: string | undefined;

if (typeof x !== 'undefined') {
  x.trim(); // works as expected
}

switch (true) {
  case typeof x !== 'undefined':
    x.trim(); // thinks x is potentially undefined
}

Expected behavior:

Narrowing types should work inside the case.

Actual behavior:

Any assertion in case seems to be disregarded

Playground Link: https://www.typescriptlang.org/play/index.html?ts=Nightly#code/DYUwLgBAHgXBDOYBOBLAdgcwgHwgVzQBMQAzdEQgbgChqUSIAKMATwAcQB7BqCAQgC8AiAHICxMmgoiAlBADe1CNAB0yFAFtGMyhAD0eiAHdOSANbwIAQ0sgoHAMZgK1AL614RlGAcALJsh4IHKKyg42IBCsHNzQ-EKi4qTkhCIwSsqq6lo6+oZgvugWcSiWbJzOaGAoVsDALPhEyVKEbkA

Related Issues:

[joscha]

Two come to mind: • readability for big amounts of cases • being able to use fall-throughs for overlapping cases

Having said that I understand it is an edge case. I came across this pattern in a bigger codebase and wanted to make a change. That’s when I noticed that the types are not narrowed in each case.

[polomsky]

I have similar usecase:

switch (true) {
    case error instanceof CustomError1:
        return error.foo;
    case error instanceof CustomError2:
        return error.bar;
    case error instanceof Error:
        return String(error);
}

And I am using this syntax in javascript because it looks better than chain of if else. Sadly can not use in typescript