Always true/false condition makes typeguard fail about typescript HOT 4 CLOSED

The first one is consistent with a regular "and", for which intersection is a good representation of what "must" be true about what's going on

    if (isString(arg)) {
      if (typeof arg === 'string') {
        const z = arg;
        //    ^?
      }
    }

The second one... I don't know what to make of this. It's possible for a string to end up there; the handling of this situation seems correct.

const s1: {} = "";
const s: Record<string, number> = s1;
t2(s);

from typescript.

The first one is wrong without the 2nd part of the condition too (and in your case too). Here without the 2nd part of the condition, it's not correctly report type of "z"

function t1(arg: Record<string, number>|number): string {
    if ( isString(arg) === false ) {
        const y = arg;
        return '';
    }

    const z = arg; // should be "never"
    return '';
}

This should work and "z" be never or it reported as unreachable.

The second one

Fair enough, this empty object issue is another unrelated issue that has been discussed here widely already, so not going to do that again here :)
So the 1st case is left as an issue?

from typescript.

z isn't unreachable in legal code, though.

const n1 = "hello";
const n2: { length: number } = n1;
const n3: Record<string, number> = n2;
t1(n3);

from typescript.

You're right, since {} is assignable to string confusingly... (e.g. using boolean|number then it works without errors)

Thanks for clarifying.

from typescript.