Union types get narrowed unexpectedly

[mmktomato]

TypeScript Version:
3.8.3 and Nightly

Search Terms:

• type inference
• union type
• narrowing
• callback
• function

Code

const fn1 = (cb: (n: number) => void) => {
    cb(0);
};

const fn2 = () => {
    let a: number | string = "";
    fn1(n => a = n);
    return a;
};

type T = ReturnType<typeof fn2>;  // T is string

Expected behavior:
I define a as number | string explicitly. So I expect T is number | string.

Actual behavior:
T is string.

Playground Link:
https://www.typescriptlang.org/play/?ts=nightly#code/MYewdgzgLgBAZmAjDAvDAFMARgLg2PMAVwFssBTAJwEpUA+GANxAEsATWlBgbwCgYBMbOgAM1ANy8AvpN6hIsBACZUGTj36CANuVgBDQqQqUYAHxjRKLMAHNVAInuTB8JOjD0Ye1WAmaBlLpElB56kjK8vFAAngAO5DAAKqoASkEhiXHkADwx8SBwrkp04gIA9GVJMCwQFlBWtkA

Related Issues:
#9998

[nmain]

Two things going on here.

a is initially typed to string:
Duplicate of #8513
My search terms were union narrowing assignment
Also see discussion in #31734

fn1(...) does not reset the narrowing on a, or renarrow a to number:
Duplicate of #9998

[mmktomato]

Thank you. I understand the situation and the background.
Fortunately my case is enough to do like:

const fn2 = (): number | string => {
    // ...
};

type T = ReturnType<typeof fn2>;  // T is number | string