Type parameter should not be assumed truthy

TypeScript Version: 3.1.0-dev.20180912

Code

function f<T extends string | undefined>(x: T): number {
    return x && 42;
}
f(undefined).toFixed();

Expected behavior:

Error -- the output may be "" or undefined.

Actual behavior:

No error. (Error at runtime.)

[niazlake]

Could you explain more how can fix this and try to understand for me.

I don't know how to fix this, but we'll likely have a fix in a few months since this was assigned to the ts3.2 milestone.

[ahejlsberg]

I'm going to mark this a duplicate of #22348. We need the ability to express "the falsy parts of T" in higher order form.