TypeScript failing to narrow field types indexed w/ const assertions

[bsuth]

Bug Report

🔎 Search Terms

const assertion
const narrow
assertion narrow

🕗 Version & Regression Information

First seen in 4.1.2, reproduced in 4.4.3 and 4.5.0-dev.20210919.

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "Type System Behavior", "Type Guards", "Things That Don't Work"

⏯ Playground Link

Playground link with relevant code

💻 Code

const test: { myfield?: number } = {};
const constField = 'myfield' as const;

if (test[constField]) {
  console.log(test[constField] + 1); // Object is possibly 'undefined'
  console.log(test.myfield + 1); // Object is possibly 'undefined'
}

🙁 Actual behavior

TypeScript is not able to narrow test[constField] from undefined|number to number

🙂 Expected behavior

TypeScript infers that test[constField] is simply number in the if block

[IllusionMH]

Looks like duplicate of #44899 and linked issues.

[easyrider]

This is also possibly a #10530 duplicate - an issue that appears frequently since 2016, the solution was already proposed as a PR but was declined.

[bsuth]

indeed this seems to be a duplicate, closing in favor of #10530