Check Typescript Promise exhaustively covered with resolve | reject

Related post, I want to statically check if a Promise is guaranteed to resolve or reject:

function foo(x: number) {
  return new Promise((resolve, reject) => {
    if (x > 30) { resolve(x+7);       }
    if (x < 20) { reject("ouch !!!"); }
  });
}

async function main() {
  try { const y = await(foo(60)); console.log(y); }
  catch (e) {                     console.log(e); }
}

main();

Can the approach for enums coverage (from this post) be extended to number coverage?

EDIT:

I guess for the very least I would like to specify the return type of the promise so an undefined is not a valid value. Something similar to:

$ cat example.ts                         
function bar(x: number): number { if (x > 30) { return x-2; }}
$ npx tsc --strict --target es2020 example.ts | grep error
example.ts(1,26): error TS2366: Function lacks ending return statement and return type does not include 'undefined'.

1. Use async functions

// ~! Function lacks ending return statement and return type does not include 'undefined'.(2366)
async function foo(x: number): Promise<number> {
    if (x > 30) { return (x + 7); }
    if (x < 20) { throw new Error("ouch !!!"); }
}

Promises are especially used in cases where you have no idea when you return something