Argument type refinement lost in binary expression

[thers]

Bug Report

🔎 Search Terms

Type refinement, binary expression, logical or

🕗 Version & Regression Information

This is the behavior in every version I tried (didn't test below v4 though), including nightly.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface SomeObj {
    foo: string,
    bar: number,
}

function test_INCORRECT(obj: SomeObj | undefined) {
    if (!obj) {
        return null;
    }

    return {
        foo: obj.foo,
        baz: true || (obj.bar === 42),
    };
}

🙁 Actual behavior

A TS2532 Object is possibly 'undefined' error is rised for obj in true || (obj... part of code, despite the fact that obj was refined to be exactly SomeObj.

🙂 Expected behavior

No error for this part of code.

[ajafff]

That's because the code after true || is unreachable. TypeScript doesn't do any type narrowing in unreachable code.

Therefore this is a duplicate of #26914

[thers]

Thanks for linking that issue, I wasn't able to find anything related as just searching for true || term gives you 300+ pages of issues!