TypeScript type guard issue

Consider following code:

interface XBase {
    a: string;
}
interface X1 extends XBase {
    v1: string;
}
interface X2 extends XBase {
    v2: string; // A
}
function isX1(arg: X1 | X2): arg is X1 {
    return (arg as Partial<X1>).v1 !== undefined;
}
function isX2(arg: X1 | X2): arg is X2 {
    return (arg as Partial<X1>).v1 === undefined;
}
function f(x: X1 | X2) {
    const x1a = isX1(x) ? x : undefined;
    const x1b = isX2(x) ? undefined : x;
    if (x1a) { console.log(x1a.v1); }
    if (x1b) { console.log(x1b.v1); }
}

This works as expected.

When the line marked A is removed, the type of x1b changes to undefined.

Why?

Using tsc version 4.9.5

> When the line marked A is removed, the type of x1b changes to undefined.
>
> Why?
>
When you remove that line, everything that is an X1 is also an X2. Remember that an object is allowed to have excess properties, so something that's an X2 must have a: string, but it could also have arbitrary other properties, including a v1 which is a string. Thus, { a: 'foo', v1: 'bar' } is an X2 as well as an X1 (as well as an XBase)

Because of this, typescript deduces that it's impossible for isX2 to return false. If you pass in an X2, then obviously your function will return that it's an X2. But if you pass in an X1 then as far as the types are concerned, it must be an X2, and so typescript expects your function will return true as well. Since it always returns true, x1b must always be undefined.

Now your actual implementation of isX2 wouldn't let a V1 slip through, but the types say it would. It's a bit ugly but you could change your return type on isX2 to specify that not only is the value an X2, but it doesn't have a v1 property.

function isX2(arg: X1 | X2): arg is X2 & { v1: undefined } {
    return (arg as Partial<X1>).v1 === undefined;
}

With that modification, it's no longer guaranteed that an X1 would return true.

In practice this type of situation doesn't come up much. Most of the time, if you're writing an X2 type it's because you want to add some distinguishing features to it, like you had before commenting out line A. And this will only come up when you don't have those distinguishing features.

Playground link