Promise.then variance issue

[chancancode]

Hello!

I understand that function parameter are unsound/bi-variant in TypeScript in general, however, I thought there was a "targeted fix" for promises at some point, so I am wondering if this is a considered a bug:

function stringOrNumber(): string | number {
  return Math.random() > 0.5 ? 'hello' : 3.14;
} 

function stringToString(s: string): string {
  return s.toUpperCase();
}

Promise.resolve().then(stringOrNumber).then(stringToString);

Playground link

[DanielRosenwasser]

Unfortunately this is expected from parameter bivariance (but certainly not desired). The fix you're referencing from 2.4 prevents the following bug:

    function toStringOrNumber(): string | number {
      return Math.random() > 0.5 ? 'hello' : 3.14;
    } 

    function stringToString(s: string): string {
      return s.toUpperCase();
    }


    let stringOrNumber = Promise.resolve().then(toStringOrNumber);
    let str = Promise.resolve("").then(stringToString);

    str = stringOrNumber;
//  ~~~ bad

[DanielRosenwasser]

Check out discussion at #18513 in the meantime.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.