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

• I havesearched for related issues and found none that match my proposal.
• I have searched thecurrent rule list and found no rules that match my proposal.
• I haveread 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 useas. I default to tell people "don't useas", 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 astring, but unsafe in astring | boolean.

This rule should only allow doingx as Type in the cases where the type ofx is assignable totype. So, the rule of thumb would be: ifconst a: Type = x is fine, thenx as Type is fine. Otherwise it's not.

Fail Cases

constx='something';consty=xasunknownasnumber;// Type `unknown` is not assignable to type `number` 🚫//-----------functionf(){returnMath.random()<0.5 ?42 :'meh';}constz=f()asnumber;// Type `number | string` is not assignable to type `number` 🚫

Pass Cases

consta='something';constb=aasunknown;// Type `string` is assignable to type `unknown` ✅//-----------functiong(){returnMath.random()<0.5 ?42 :'meh';}consth=f()asnumber|string|boolean;// Type `number | string` is assignable to type `number | string | boolean` ✅//-----------constperson={name:'Chuck',lastName:'Norris'asstring|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.