typescript-bug.ts

interface Cat {
    readonly name: string | undefined;
}

function catPrinter(cat: Cat) {
    if (cat.name === undefined) {
        return;
    }

    console.log(cat.name.trim());
}

function delayedCatPrinter(cat: Cat) {
    const cat2 = cat;

    if (cat2.name === undefined) {
        return;
    }

    setTimeout(() => console.log(cat2.name.trim()), 100);

    // ...
}

function delayedPrinter(cat: string | undefined) {
    const cat2 = cat;

    if (cat2 === undefined) {
        return;
    }

    setTimeout(() => console.log(cat2.trim()), 100);

    // ...
}