Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
• My proposal is not a "formatting rule"; meaning it does not just enforce how code is formatted (whitespace, brace placement, etc).
• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

One of the hardest thing to explain to people not experienced in TS is when it's ok to use as. I default to tell people "don't use as", and I constantly have to deal with that in code reviews. I'd love to have a rule that only allows type assertions that widen a type. For example, as unknown is always totally ok. as string | number is safe on a string, but unsafe in a string | boolean.

This rule should only allow doing x as Type in the cases where the type of x is assignable to type. So, the rule of thumb would be: if const a: Type = x is fine, then x as Type is fine. Otherwise it's not.

Fail Cases

const x = 'something';
const y = x as unknown as number; // Type `unknown` is not assignable to type `number` 🚫

//-----------

function f() {
  return Math.random() < 0.5 ? 42 : 'meh';
}
const z = f() as number; // Type `number | string` is not assignable to type `number` 🚫

Pass Cases

const a = 'something';
const b = a as unknown; // Type `string` is assignable to type `unknown` ✅

//-----------

function g() {
  return Math.random() < 0.5 ? 42 : 'meh';
}
const h = f() as number | string | boolean; // Type `number | string` is assignable to type `number | string | boolean` ✅

//-----------

const person = {
    name: 'Chuck',
    lastName: 'Norris' as string | undefined // Type `Norris` is assignable to type `string | undefined`  ✅
}
person.lastName = undefined; // this is fine because of the type assertion (just showing why we'd want the `as string | undefined`)

Additional Info

Here's a link to a TS playground with the passing and (proposed) failing cases.