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.)

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

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.

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